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Crossing the Paradigmatic Divide: Coming to an Evolutionary Understanding of Personality Variation


Steven C. Hertler

[Journal für Philosophie & Psychiatrie, May 2015, Original paper]




Evolutionarily oriented personality researchers expect and accept heterogeneity within human personality. During the past decade, such evolutionists have demonstrated that personality variation is actively created by natural and sexual selection.  Nevertheless, some practitioners and researchers, because they seem not to have fully understood the source and implications of evolved variation, continue to effectively operate under two assumptions: First that personality and its variation are primarily environmentally instilled; second, that there is an optimum or ideal personality, deviation from which implies abnormality or pathology.  This paper, exploring concepts such as adaptive diversification, environmental heterogeneity, and negative frequency dependent balancing selection, clarifies, consolidates and disseminates the knowledge of the evolutionarily oriented personality researcher, as it pertains to personality variation, in a manner that is accessible to clinicians and general clinical researchers. As discussed, in addition to fostering a philosophy of personality and personality disorders by delineating relativistic and absolute standards of dysfunction, the evolutionary logic of heritable personality variation also facilitates an understanding of gene by environment interactions, such as reactive heritability and experience producing drives.



The understanding of personality, herein operationally defined as traits and trait patterns as described by McCrae & Costa (1995), has been historically dominated by two assumptions: (1) It has been widely assumed that personality was primarily environmentally instilled, with parental influence being the most instrumental of all environmental factors; (2) also, there is the concept of a healthy personality; a desirable ideal, deviation from which increasingly indicates pathology. 


The first of these assumptions, the idea that one’s personality is principally a reflection of one’s environmental past, is presently referred to as psychogenic character formation, in contradistinction to physiogenic (Gillespie, 1928; Blaney & Millon, 2009), which denotes constitutional or genetic sources of character formation.  In the main, personality paradigms have emphasized environmental factors to the relative exclusion of biological factors, and so in the main, most personality paradigms can be described as psychogenic (Zuckerman, 1991). Personality is shaped just as one rock buffets against others in a river (Mallinger, 1984) or is “seen as series of transformations through continual interrelations with other human beings” (Frank, 1949). Psychoanalytic formulations attributed personality variation to predominately environmental causes, focusing most particularly on parenting (Gay, 1989; Horney, 1937; Mallinger, 1984). Similarly, within a behavioral paradigm, personality variation comes of variation in reinforcement history; as John Watson quipped, “give me a dozen healthy infants, well-formed…”  Like the first two, the third force in psychology, Carl Rogers and his humanistic psychology, also stressed environmental and parental influence (Rogers, 1957). Truly, psychogenic character formation is an idea that is as timeless as it is intuitive, as Dumont (2010; page 9) describes:

"Plato also repeatedly reaffirmed his conviction that the child’s adult character was shaped in the earliest years of its life by the care and instruction given by parents and teachers, a principle that would later come to be associated with Rousseau, Freud, and many twentieth-century psychologists."

Though it is at least as old as Plato, and emblematic of Rousseau and Freud, psychogenic character formation is perhaps most quintessentially Lockean.  The Lockean child is born a blank slate ready to receive, through experience and instruction, character and traits (Locke, 1689/1996).  Moreover, this is an idea with lasting influence and contemporary force. Not only parenting (Belsky, 1984; Mallinger, 2009), but reading (Russell, 1952), education (Heffernan et al., 1952; Mei & Jiahui, 2010; Yao, 2010), psychosocial factors (Husain & Rasouli, 2009) and psychological treatment (Barondes, 2009) are among the supposed sources of character construction. 


Second, there has long been the idea of an individual optimum, whether it is described as the normal personality (Shoben, 1957; Strack, 1999; Cloninger, 2012; Norman, 2013) or the healthy personality (Burnham, 1932; Horney, 1937; Erikson, 1950; Senn, 1950; Hymes, 1963; Jourard, 1963; Yamamoto, 1966; Cattell, 1973;, Lazarus, 1975; Schultz, 1977; Hershenson, 1982; Seijts, 1998; Miller, 1998; Day & Rottinghaus, 2003; Cloninger, 2012; Wright, Ansell & Pincus, 2013). Individual optimums are assessed via commercially available instruments, such as the Healthy Personality Inventory (Betz & Borgen, 2010), research-based measures, such as the Healthy Personality Scale (Yu, Zheng, Yan, & Wan, 2008), general measures, such as Personal Orientation Inventory (Hightower, 1988), and clinical measures, such as the Minnesota Multiphasic Personality Inventory (Quirk, Christiansen, Wagner, & McNulty, 2003). This idea, the notion that there is an ideal personality, has elicited (White, 1973; Alarcon & Foulks, 1995) and is eliciting (Leising, Rogers & Ostner, 2009; Betz & Borgen, 2010) some thoughtful critiques that note cultural and operational problems, though it is more often applied than analyzed. Most recently, the American Psychiatric Association (2013) has explicitly acknowledged and defined the role of the healthy personality, stating that it is composed of the following traits: Emotional stability, extraversion, agreeableness, conscientiousness, and lucidity.


Finally, it is important to recognize the relationship between these two ideas. In the first place, if character is preeminently malleable, it can be hammered into an ideal. In testament to this assertion are the myriad treatments describing movement towards a personality ideal (Glasser, 1950; Chiang & Maslow, 1969; Hershenson, 1982; Walsh, 2003), some of which are associated with classical traditions like psychoanalysis and humanistic psychology (Jourard, 1974; Rogers, 1957) and some of which are associated with progressive traditions such as growth psychology (Schultz, 1977) and positive psychology (Prager, Shirvani, Garcia, & Coles, 2013). Therefore, the first assumption serves the end of the second; and jointly, these assumptions promote ideals of change, growth and treatment. More to the point, the second assumption is largely dependent on the first. While psychogenic personality formation is logically possible with or without the notion of an ideal type, the reverse is not true. Specifically, while assuming character to be determined psychogenically, one might reasonably assert that personalities are as various as the environmental histories that form them; and then one is free to say that both the variation in personality and environment is, or is not, indicative of problems or pathology. However, one is put in a bind when asserting that personality variation is largely inherited, while trying to maintain, and scientifically explain, the notion of an ideal type. This then begs the question: If personality varies so often and so much away from the ideal, why are dysfunctional types so persistent and so prevalent? 


The purpose, structure and aims of the present article

The two above-described assumptions are vestiges of Lockean thought. Nothing could have been at once so intuitive and so inaccurate. And it is because of this intuitive appeal that these concepts endure, in spite of evidence to the contrary.  It is not that these concepts are unequivocally incorrect or thoroughly useless; it is just that, to the degree to which they have been promulgated and in the ways in which they have been applied, they conspicuously conflict with prevailing data. While acknowledging that environmental and genetic influences are both important, and that these forms of influence combine dynamically to create personality and its variation, a personality theorist learned in evolutionary literature and behavioral genetics research, rather than seriously entertaining these two assumptions, understands personality to be largely inherited behavioral variation directly under selective pressures. Nevertheless, this view, animate in many minds, and implicit in many manuscripts, has only been articulated comprehensively and pointedly in rare instances (Penke, 2007; Penke, Dennisen, & Miller, 2007; Buss & Hawley, 2011). 


Targeting general practitioners, clinicians, psychologists, diagnosticians, and researchers, that simply acknowledge the heritability of personality without truly understanding its source or insinuating its implications into their explanatory framework, the present paper is meant to consolidate and disseminate an evolutionarily oriented explanation of personality variation that can replace, qualify and contextualize antiquated notions of psychogenic character formation and an ideal personality. Concepts such as adaptive diversification, intraspecific character displacement, and negative frequency dependent balancing selection are reviewed in an accessible manner bringing one to the view that personality diversity is actively created by natural and sexual selection, so one should expect and accept heterogeneity within human personality. This paper then does not call for a paradigm shift; rather it articulates one already made within the literature, with the end of recapitulation at the level of the individual reader. Finally, in addition to considering how an evolutionary view of personality variation is contrary to the diagnosis of personality extremes, the discussion attempts to ease the transition to an evolutionary paradigm, firstly by showing that these evolutionary concepts, seemingly so alien, are familiar and intuitive and secondly by describing the continuing contributions of environmental variables.


Heritability of Personality and Making the Transition

Inspired by the pioneering efforts of Francis Galton (1869), the field of behavioral genetics has amassed evidence through decades of controlled twin studies (Floderus-Myrhed, 1980; Tellegen, et al.,1988; Jang, et al., 1996; Riemann et al., 1997; Bouchard & Loehlin, 2001), now consolidated by a multitude of meta-analyses (McCartney, et al., 1990; McGue, Bacon, & Lykken, 1993; Rhee & Waldman, 2002; Johnson et al., 2008).  Data deriving from these studies broadly converge with what is known about animal personality research (DeYoung & Gray, 2009; Carere & Maestripieri, 2013); research which has found similar rates of heritable behavioral variation in insects (Wolf & Weissing, 2012), fish (Reale, 2007), reptiles (Lopez et al., 2005), birds (Dingemanse et al., 2002), undulates (Van Oers, 2005), and primates (Weiss, King, & Figueredo, 2000). With an important role for heritable personality variation being irrefutably confirmed (McCrae, 2009), debate and research now centers on establishing the ultimate proportions of heritable and environmental personality variation (Krueger, South, Johnson, & Iacono, 2008). Defined as “the ratio of variance from genetic sources to total variation in the trait,” or “the proportion of total trait variation linked to genetic variation,” heritability estimates of personality traits generally stand at .50 (Krueger & Johnson, 2008; page 288), though estimates do range “from 40 or 50 percent up to 80 percent, depending on trait and method” (DeYoung & Gray, 2009; page 324).   


In addition to establishing the proportional influence of nature, behavioral genetics can parse nurture into shared in-families components and non-shared extra-familial components. These two divisions of the environment are neither equally influential, nor divided unequally upon expected lines.  Citing several sources (Beer, Arnold, & Loehlin, 1998; Eysenck, 1990; Goldsmith, Buss, & Lemery, 1997; Zuckerman, 1997), Dumont (2010; page 123) states that “abundant evidence exists” demonstrating that “the shared environment of a stable home and a consistent pattern of childrearing exert little influence in shaping the temperament of the child.” This is reflected in the three laws of behavioral genetics distilled by Turkheimer (2000): (1) “all human behavior traits are heritable"; (2) “the effect of being raised in the same family is smaller than the effect of genes"; (3) “a substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families.” In light of these findings, psychogenic theories are now known to have erred, twice over. Collapsing across theories, it can be said that environmental influence has been accentuated above and beyond genetic influence; and additionally that familial environmental influence has been accentuated above and beyond extra-familial environmental influence. In this way, these theories are doubly wrong in that they took the smallest slice of variance in the first cut, and then again in the second, being left with a sliver which they championed to the detriment of the pie at large. This highly counter-intuitive finding, reflected in Turkheimer’s (2000) above quoted second law, contradicts what has been called, the wisdom of the ages (Dumont, 2010; page 124). 

Unfortunately, evidence of personality heritability is as staunchly resisted as it is firmly established. In addition to citing fears of biological determinism and the inability to escape intuitive explanations, Dumont (2010; page 115) formally outlines four reasons for the persistence of psychogenic explanations of character formation: (1) there is the momentum of tradition; momentum by which personality psychologists are still much more comfortable with social scientific data as opposed to natural scientific data; (2) many of the scientific methods and technologies that are used to support nature oriented explanations of personality are newly developed, as are the data that they obtain; (3) under the influence of funding, policy and the inclination to clinically treat, psychological research, and the theories that derive from it, are often skewed towards emphasizing controllability; (4) finally, “fluid environmental explanations of personality,” as opposed to “fixed biological ones,” are consistent with individualism, choice, self-creation and other highly prized Western values. 


The notion of tradition imparting momentum is a particularly Kuhnsian (1996) concept, for it at once demonstrates that scientific data can be subject to unscientific inattention, and that scientists can be biased by ideological preference.  Both points are evident in McClintock’s long-neglected data on transposition, which showed, contrary to prevailing dogma, that genes can propagate horizontally within generations, not just vertically across them (Nathaniel & Compfort, 2009). Momentum, in this case the momentum of shared within-families psychogenic explanation, can certainly be perpetuated by individuals whose training, career and publication record is wedded to an ailing thesis.  Irrespective of such factors, heritable personality variation is more or less acknowledged by all, but only influential for those who have dwelt upon its implications. And so it is in this sense that paradigmatic change is called for; it is because there remain are large segment of clinicians that practice, professors that teach, and authors that write, which may occasionally recall, briefly note, and sparingly report on personality’s heritability, without apprehending its source and resultant implications. Dumont’s second point is also well made, though the new technologies and complex data he refers to are more relevant to genetics studies that proximately specify behaviorally relevant genes, than to evolutionary theory that ultimately explains behaviorally relevant genetic variation.


As momentum wanes and data waxes, desire for control and fears of determinism may continue to serve as stalwart impediments. Really two sides of the same coin, these third and fourth of Dumont’s obstructions are, in essence, a subset of the more general resistance against ideas, like the heliocentric theory advanced by Copernicus and pressed by Galileo (Cohen, 1985), that subordinate, demote and remove the façade of human specialness.  However, when explanatory of human behavior, such ideas have added sting, and are greeted with extra vitriol, because they humble hubris by undermining self-sovereignty. Psychoanalytic theory, with its unconscious determination of behavior (Meissner, 2005), and operant conditioning, with its implications of environmental determinism (Bandura, 1978; Delprato & Midgley, 1992), show that threats against self-determination can come from any quarter; yet now neither evokes resistance as does hereditary explanation, with its implications of biological determinism. Just like evolution when generally applied to human behavior within the disciplines of ethology and sociobiology is resisted for its putative deterministic implications (Alexander, 1976), so will be any hereditarian explanation of personality variation in that it limits controllability and constrains fluid environmental explanation. In either case, control and choice is diminished, and so human hubris is humbled. With half the variance in personality being ‘genetically determined’, the parent, therapist and self are divested of half their influence.  To some extent, such trepidation can be palliated by properly confining personality within a trait based operational definition, as has been done, and to some extent such trepidation can be palliated by describing how the remaining environmental variance interacts with genetic determinants of personality variation, as will be discussed.  Though, as reviewed by Dumont, there are these four operative impediments to assimilating hereditarian explanations of personality variation, there is perhaps one main method by which to remove those impediments; namely the clear explanation of available evidence. 


Within the Province of Evolutionary Theory: Early Forays

In addition to evolutionary explanation being initially focused on the explanation of universal adaptations driven to fixation within the population (Penke, 2011), the dominance of psychogenic theories seems to have inhibited personality’s evolutionary explanation.  This is because only heritable traits are subject to evolutionary change; without a genetic component selective pressures cannot modify a trait from parent to offspring, across several generations. So only after behavioral genetics studies established its heritability, were personality and its variation thrust into the province of evolutionary explanation.  It took some time, however, before evolutionary theory made sense out of personality variation-initial theories missed its significance. First, selective neutrality, promulgated by Tooby and Cosmides in the early 1990s (Buss, 2009), suggests that selective pressures do not operate on personality and individual differences.  Personality and individual differences are, as the name implies, selectively neutral.  In other words, selective neutrality attributes personality variation to random noise that, although it is phenotypically expressed and genotypically represented, is not relevant to fitness, and so not acted upon by evolution.  This theory would hold that the introvert and the extravert, the agreeable and the disagreeable, those open to experience and those closed to it, are equally likely to survive and reproduce because the “phenotypic consequences” of personality are “…completely unrelated to fitness in any environment” (Penke et al., 2007). Second, mutation load proposed that Darwinian selection is ever pressing towards an ideal personality type that is most fit. Mutations, some of which will have behavioral consequences, arise through genetic copy errors, among which are single nucleotide polymorphisms (otherwise known as SNPs or point mutations) that produce a substitution in a single base pair (Verweij, et al., 2012); indels (formed from in for insertion and del from deletion) that insert or delete nucleotides from a genetic sequence (Choi et al. 2012), and copy number variations (CNVs) which, rather than inserting or deleting a nucleotide, delete or repeat relatively larger sections of the genome (Penke, et al., 2007). As per mutation load, if the mutation is pernicious, it will be eliminated from the population, but if it confers advantage, it will be fully incorporated into the population. Time is simply needed to eradicate or fully incorporate behaviorally relevant mutations. The existence of personality variation therefore simply represents the failure of natural selection to quickly and completely actualize this homogenizing tendency (Penke et al., 2007).  


For some time now it has been understood that personality is not imperceptible noise unnoticed by natural or sexual selection; consequently selective neutrality is an “implausible explanation for heritable personality differences” (Penke, Dennisen, & Miller, 2007). Unlike selective neutrality, mutation load theory does stoop to notice personality variation. Notwithstanding, it sees evolution as ever homogenizing personality towards some ideal state, and diversity as some temporary aberration that mutationally arises and is eventually snuffed out. Though these mechanisms are different, both selective neutrality and mutation load simply tolerate trait variation, explaining individual differences as either inconsequential variation or fugitive variation. Neither theory really addressed what is now seen more clearly; namely that “human personality traits influence outcomes in all areas of life” (Penke et al., 2007).


Towards Adaptive Diversification

Only after evolutionary theory was applied much more rigorously and fully did it become clear that personality diversity, instead of being tolerated or discouraged, was actively created by evolution. The chain of ideas unfolds, if not historically, then conceptually thus: Sympatric species, species inhabiting the same environmental space, are potential competitors.  The more one species is physiologically and behaviorally similar to another, the greater will be the likelihood and intensity of competition. Similar sympatric species often undergo extinction, specialization, or at least divergence within the area of overlap (Schoener, 2009). There is, in fact, a limiting similarity (Schoener, 2009) mathematically described by Hutchinson’s Ratio, which specifies the “minimal degree of separation needed to decrease competition and achieve niche separation between closely sympatric species” (Cachel, 2006; page 210). The limiting similarity and Hutchinson’s Ratio are, in turn, relevant to the concept of adaptive radiation, the punctuated diversification of one species into several species. Adaptive radiation is often precipitated by dispersal, but it can also be elicited by the sympatric competition described above.  In either case, a single ancestral species, which is usually comprised of a relatively generalist type, diverges into several species, which are usually comprised of relatively specialized types (Schluter, 2000).  Species produced by recent adaptive radiations, while extraordinarily similar across the great majority of traits, are then extraordinarily dissimilar only on the trait, or small set of traits, responsible for speciation (Travisano, 2009). Darwin’s finches, with their widely divergent beaks, are the iconic example of this rather ordinary process (Schluter, 2000).


What is presently significant about adaptive radiation is that sometimes this process is initiated without culminating in speciation; in this case it is known as adaptive diversification (Doebeli, 2011; Bell, 2009). In other words, the process of adaptive radiation with its creation of interspecific variety is only quantitatively different from the process of adaptive diversification with its creation of intraspecific diversity; adaptive radiation is to macroevolution what adaptive diversification is to microevolution. Through the process of adaptive diversification, some species members are driven to exploit different resources, behave differently, or develop alternative morphologies under intense within species competition. Personality variation within humans is simply a behavioral manifestation of this common process. 


Identifying Mechanistic Explanations

Thus far, however, this only suggests that human personality diversity is a product of competition. So it tells one that diversification is adaptive, but not how or why.  Balancing selection is what ultimately and mechanistically explains these latter two questions, telling us why diversification is adaptive and how it alleviates competition. The traits subject to balancing selection, instead of being shaped towards some best solution, are balanced between competing solutions:

"…the phenotypic and genetic characteristics that are typically found in studies of personality traits (like those in the FFM [five-factor model]) suggest that balancing selection is maintaining the genetic variance in most (if not all) personality traits. Balancing selection can favor different traits in different social or non-social environments." (Penke, Denissen & Miller, 2007)

Balancing selection itself comes in a number of formats, as Penke et al. (2007) explain: (1) overdominance1, also known as heterozygous advantage; (2) antagonistic pleiotropy2(3) environmental heterogeneity; (4) frequency-dependent selection. Of these various forms of balancing selection, the first two are not regarded as plausible mechanisms by which personality variation could be explained or maintained; in contrast, the second two jointly accomplish this. The third mechanism, environmental heterogeneity, explains trait variation as a consequence of environmental variation (MacDonald, 1995) and is thereby a mechanism capable of explaining why variation is adaptive in the first place.  The fourth mechanism, frequency dependent balancing selection, explains precisely how that variation, once established, is maintained (Penke, Denissen & Miller, 2007).


Creating Variation: Environmental Heterogeneity

Referred to in some literature as geographic heterogeneity (Doebeli, 2011), environmental heterogeneity, as applied to personality, is premised on the assumption that heterogeneous environments can create heterogeneous selective pressures, which in turn can create heterogeneity within the traits and personalities of persons (Penke, 2007). Though conceptually straightforward, the nature of environmental heterogeneity is dependent on operational definitions of the ‘environment’; a fact that introduces complexity. First, environments can be construed specifically as macroscopic and cross-regional.  In this view, the environment can change as a function of abiotic variables including temperature and seasonality, or biotic variables including parasitism and predation, any of which might alter the selective regime thereby introducing heterogeneity into personality (Harcourt, 2012). So, personality traits, types and means can vary as a function of the abiotic and biotic selective pressures that vary as a function of locale, latitude and altitude (Templeton, 2014). These macroscopic environmental differences may, for instance, be especially well positioned to explain some portion of the population mean trait variation described by cross-cultural psychologists (McCrae, 2001; Allik & McCrae, 2004; Hofstede & McCrae, 2004; McCrae et al., 2007; McCrae, 2009).  Yet, as Templeton (2014; page 807) states, the power of this macroscopic variety of environmental heterogeneity may not explain the lion’s share of genetically imparted personality diversity:

"Isolation by distance ensures that human populations are genetically differentiated from one another, and local adaptation ensures that some of these differences reflect adaptive evolution to the environmental heterogeneity that our globally distributed species experiences. However, most of our genetic variation exists as differences among individuals, with between-population differences being very small."


Second, and partially explicative of the within population personality variation described above, environments can be construed as more local regions containing abiotic and biotic selective pressures that are homogeneous with respect to space, but heterogeneous with respect to time. In this view, local environments buffet organisms to and fro, favoring one trait, type, or optimum and then another, as, with time, they are changed by species introduction and species extinction, immigration and emigration, cycles of flood and drought, feast and famine, warm and cold. 


Third, and least obviously, but most importantly, the environment can be conceptualized as social, and its variation can be conceptualized as socially generated (Gutiérrez, et al, 2013; Réale & Dingemanse, 2011). By this definition, the environment can be heterogeneous even as both space and time are held constant. Perhaps most capable of explaining “the great diversity in personality among interbreeding persons in a contiguous locale,” this specific variety of environmental heterogeneity is, in some literature, referred to as the coral reef model (Sherman, Figueredo, & Funder, 2013; Figueredo et al., 2011). Originally developed to explain animal personality diversity (Figueredo, Woodley, Brown, & Ross, 2013), the coral reef model, now routinely applied to humans, describes a variety of “social microniches to which different personality and life history variants are differentially adapted” (Sherman, Figueredo, & Funder, 2013). When encompassing the social environment, environmental heterogeneity then expands to generally include intraspecific competition, and so sexual selection together with additional classes of natural selection. When taken together and in construing the environment in these three ways, environmental heterogeneity is the best positioned mechanism for explaining the observed variation  in personality; it, in quintessence, is a mechanistic account of the general principle described above as adaptive diversification. Diversity in personality can be said to be created by diversity within the environment-across space, through time, and within social populations.


Maintaining Variation: Frequency Dependent Balancing Selection

While environmental heterogeneity initially creates personality diversity within human populations, it is frequency dependent selection, most formally negative frequency dependent balancing selection3, which most convincingly, powerfully and fully explains the maintenance of that variation within interbreeding populations, unbarred by distance or geography. Personality traits regulated by negative frequency dependent balancing selection become less adaptive as they become more common.  In other words, fitness and frequency are negatively correlated. Thus, uncommonness is a virtue unto itself.  Because this concept is less intuitive than environmental heterogeneity, illustrations are necessary;


Smoke begins to replace breathable air, temperature increases, someone in the crowded room cries fire! Panic causes a rush towards the exit; inevitably the mass of people push, crush and trample towards the doorway through which they entered. Yet, there is a back door, an emergency exit and several windows, any of which would allow safe egress. Still, finding a back door might require cognitive flexibility, which is in short supply in panic inspiring situations; exiting through a window seems to require dexterity; finding an emergency exit might depend on unusual awareness of one’s surroundings. All such solutions serve as more difficult and unlikely alternatives to the obvious means of escape: running through the main entranceway. However, as the obvious exit becomes increasingly obstructed, the advantage of pursuing an alternative exit becomes increasingly recompensed, especially to one who is, for example, spry enough to jump through the window, but too small to push through the crowd. Each means of egress can only be assigned an adaptive value relative to the number of people attempting to use it.  Only if a horde of affrighted persons block the main exit, will it be adaptive to seek a back door. Likewise, only when both the main exit and the back door become obstructed, will the risk of jumping out of a window become adaptive. Like Buss’s (2009) analogy describing commuters taking alternate routes in response to a traffic ridden thoroughfare, the above described scenario illustrates the mechanics by which diversity is maintained within a species. Getting out of the burning building or getting to work on time are analogies, of course, to surviving and reproducing. 


As these analogies demonstrate, there are instances in which it can be adaptive to jump out of a window instead of going through a main exit, or quicker to take a circuitous backroad, rather than a straight and smooth freeway. Likewise, when traditional paths towards resource acquisition, such as dominance and aggression or honesty and conscientiousness, are pursued by a majority of the population, alternatives, such as charm and alliance formation or manipulation and deception, become viable alternative strategies; or as Penke (2011; 256-257) states:

“there will always be some niches for cheaters and co-operators (related to the personality traits of psychopathy and agreeableness), for risk-takers and hesitators (related to sensation seeking), for liberal and conservative attitudes (related to openness to experiences), for long-term planners and short-term opportunity seekers (related to conscientiousness), for monogamous and promiscuous individuals (related to sociosexuality and extraversion), for aggressive hawks and peaceful doves (related to extraversion and agreeableness).”

Thus, the adaptive value of any trait or type must be judged in a relative, rather than in an absolute sense. So, no one personality can ever become the one and only personality.


The Power and Ubiquity of Adaptive Diversification

The concept of adaptive diversification, derived through environmental heterogeneity, and regulated by negative frequency dependent balancing selection is well supported by simulations; as Hawley (2011; 63) states: “…game theoretic models and field research have demonstrated unequivocally that multiple morphs can co-exist in a population, each taking advantage of a slightly different environmental niche.” Experimentally induced competition has also generated adaptive diversification in bacterial populations (Rauser, Mueller, Travisano, & Rose, 2009; Pfennig & Pfennig, 2012). Still further evidence comes from empirical observation of animal populations that derive structural and behavioral diversity through intense intraspecific competition; diversity which is then maintained by frequency dependent selection. Uta stansburiana, the male side-blotched lizard, though a single species, has three distinct behavioral and physical forms: The orange throated lizard is actively aggressive, stealing mates by force; the blue throated lizard is highly territorial, exhibiting aggression only as needed to protect his person, place, and mates; and the yellow throated lizard is furtive, sneaking copulations rather than fighting for them (Bodine, 2003). So, there are three distinct morphs, all competing for the same goal of mating opportunities, but doing so with distinct strategies and correspondingly distinct physical forms (Hawley, 2011). In five of the six years of observing these competing lizard factions, the aggressive orange throated lizards were superior, meaning that orange lizards were more successful than blue and yellow rivals in securing mates and producing offspring (Bodine, 2003). But here is the important point: The dominance of orange throated lizards cannot drive the other two morphs from representation within the population. As this dominant strategy waxes, its success wanes. If there were, by happenstance, an abundance of dominant orange throated lizards, all attempting to rove and mate via dominance, the territoriality of the blue throated lizard and the stealth of the yellow throated lizard would become more adaptive, thereby correcting the imbalance. To this end, using a natural population of guppies, Olendorf et al. (2006) has shown that preexisting morph proportions reestablish themselves after experimental disruption.


The side blotched lizard is a celebrated example of negative frequency dependent balancing selection, but it is by no means a select example. As reviewed previously (Hertler, 2015), and as seen in Appendix I of this paper, diversity is maintained in many other animal species by this mechanism: Bluegill sunfish (Buss & Greiling, 1999), salmon (Kenrick et al., 2002), small marine isopods, swordtails (Gross, 1996), cichlid scale-eating fish (Stearns & Hoekstra, 2005), damselflies (Sherrat & Wilkinson, 2009), dragonflies (Sinervo & Calsbeek, 2010), guppies (Olendorf  et al., 2006), fig wasps (Hardy, Goubault & Batchelor, 2013), scarabs, dung beetles (Snell-Rood & Moczek, 2013), crossbills (Benkman, 1996), Gouldian finches (Pryke et al., 2007), orangutans, and arctic char (Cachel, 2006). Furthermore, Pfennig & Pfennig (2012), in tables 5.1 and 5.2 of their book Evolution’s Wedge, collect over forty examples in which resource and mating competition selected for divergence into distinct types, morphs or strategies.  It seems that in all these instances, (1) competition selects for within species diversity, (2) evolved diversity is adaptive in that if provides partial competitive release, and (3) the diversity of types is regulated by negative frequency dependent balancing selection. Human personality variation might have been added to Pfennig and Pfennig’s table, as it is simply part of this ubiquitous natural process.



As did Max Planck, Kuhn (1996), pessimistically remarked that paradigm shifts ordinarily occur between generations, not within them. Tradition and intuition can certainly overpower data. Thus far, it has in the present instance. Moreover, there is reason for pessimism, as it is particularly difficult to make the transition advocated herein. This is because it is not a matter of transitioning from one psychological paradigm to another; it involves concepts and categories from distinct disciplines, such as evolutionary biology and behavioral genetics that can appear technical and imposing. Of course patient study of these disciplines, with their psychologically relevant insights, will strengthen one’s understanding of human nature generally and of personality particularly. However, it is not necessary to devote years of patient study to come to an evolutionary understanding of personality variation. In keeping with the goal of promoting and easing paradigmatic transition, it should be made clear that adaptive diversification and the proportional regulation of competing types by negative frequency dependent balancing selection, for all their ostensible complexity and apparent foreignness, are concepts that all are exceedingly familiar with. These are precisely the same blind, bottom-up forces that drive and regulate capitalistic economies (Frank, 2011).    


Consider the modern city, dense and diverse.  It is not populated by hundreds of general stores, selling a smattering of common conveniences. Rather the populous city has an array of specialty stores, some selling fruit, meat or vegetables; some selling tobacco, liquor and smut; some selling books, movies and games; some selling crafts, supplies, and tools. All business owners share the common end of making money, but they make their money in different ways. Each type of business exploits a certain market niche. Each niche is only so big and will support only so many businesses. When a niche is overexploited, all businesses within that niche suffer economically and some fold under the pressure; when a niche is underexploited, businesses entering that niche will grow exponentially, and others follow in imitation with the effect of moderating gains. Inevitably, each market niche becomes saturated and stays saturated. The city will have about as many of each kind of store as it can support. One store too many will be driven to bankruptcy under a process of selective attrition; one too few and an entrepreneur will be guided by Adam Smith’s invisible hand to open another. Both in the case of personality and business, diversity rather than homogeneity is adaptive or remunerative; and both in the case of personality and business, proportions are regulated by negative frequency dependent balancing selection, which makes uncommonness a virtue. So, just as we have the grocer, the butcher, the baker, and the policy maker, we have the dominant, the diplomatic, the shy, and the ingeniously sly.


In Ivan Turgenev’s Fathers and Sons, a young Arkady Kirsanov, fresh from the University of Saint Petersburg, and under the influence of his irreverent anarchist chum Bazarov, takes the iconic poetry of Pushkin out of his father’s hands and replaces it with Ludwig Büchner’s Stoff und Kraft, a modern materialist treatise on Force and Matter. The then extreme arguments of Büchner’s work, advocating a materialist view of soul and self, is now a standard view among life scientists.  Arkady’s father, Nickolai, however, is benighted by the novelty of materialism, but his dismay is greatly exacerbated by the loss of Pushkin. He feels that he cannot understand the new work, though he is ready to accept that it might be true; still he cannot conceive why he would have to give up Pushkin. As Nickolai’s consternation shows, giving up the old is often the hardest part of accepting the new. In the present case, it is necessary to clearly state that Stoff und Kraft can be read alongside Pushkin; that is, while much of personality variation is explained evolutionarily, environmental influence accounts for, by most estimates, half of the variance. Even researchers such as Dumont (2010) that attribute up to 70 percent of personality variation to genetic origins, attribute 30 or more percent of personality variation to environmental origins. This is no mean portion to disdain. Even were these more extreme estimates borne out, it is relevant to note that psychologists are routinely satisfied with retaining factors which explain thirty percent of the variation in a study, or tests which capture thirty percent of the variation in a patient characteristic.  So when the genetics of personality variation are discussed, all understand their influence to be a matter of degree (Bouchard & Loehlin, 2001). And so, in explaining personality variation, evolutionists do not discount nurture, even when they emphasize nature (MacDonald, 1995; McCrae et. al., 2000; Gosling, 2001; Buss, 2008; Michalski & Shackelford, 2010). 


Also, it should be clear, despite expressions presently and commonly used from convenience, that the proportional influence of nature and nurture is not statically divided, but combines reciprocally and dynamically to make adult personalities (Rodgers, Rowe, & Li, 1994; Saudino, 1997; Rose, 2001; Pinker, 2004; Lynch & Kemp, 2014).  Nature works via nurture (Ridley, 2003) to generate some of the most interesting developmental personality processes: Indirect or reactive heritability, gene-environment correlations, social niche specialization and epigenetic processes (Block, 1995; Scheiner, 2006; Krueger et al., 2008; Keller, 2010; Rozanov, 2012). Also known as genotype-specific habitat selection or experience producing drives, gene-environment correlations, for instance, imply dynamism in that they show environments to be, not randomly experienced, but actively sought for their ability to conform to genetically conferred traits (Penke, 2011). Thus it is, whether studying animals or humans, that the environment is “to a large extent, determined by the behavior of the individual, for example, as a result of dispersal and migration, movement patterns, habitat choice, feeding strategy, predator avoidance, mating strategy, or social behavior” (Wolf & Weissing, 2012). Thus, making the paradigmatic transition herein described entails recognizing the foundational role of evolution in creating personality variation, but does not entail rejecting the important role of the environment in augmenting that variation.  


Finally, while the continuing role of environmental influence is clear, the notion of an ideal type is not. Unless greatly qualified, the idea of an ideal personality is logically incompatible with the idea of adaptive diversification.  To retain the former while adopting the latter, one must acknowledge that the healthy ideal that they promote is a culturally relativistic heuristic. Even were there an optimum trait value, it would not follow that deviations from that optimum were inferior.  To this end, and to understand this contention, consider the writings of Figueredo et al., (2011; 213):

“Selective pressure for a species-typical monomorphism at this optimum creates a centripetal force against substantial individual variation, as opposed to a selectively neutral adaptive plateau (a range over which the variants have equal fitness). This clustering of the entire population at the ORD [optimal response disposition] produces intense social competition at the center of the distribution and relatively reduced social competition at the tails of the distribution. Reduced competition generates disruptive selection for individual differences as a centrifugal force, partially counteracting directional selective toward the ORD.  The competitive release experienced by individuals at the tails of the distribution compensate for the cost of deviation from species-typical norm of response.”


The examples of the burning building and the crowded freeway described previously illustrate this very point. The main exit might be the fastest way out of the building, and the freeway might be the quickest way to work, hence providing some sense of optimum, but the main exit and the freeway, being optimums, will inevitably be saturated. With reference to the above-quoted passage, these strategies would fall at the center of the distribution in which there is intense competition.  Leaving through the back door or taking local roads to work are viable alternatives precisely because they fall at the tail ends of the distribution in that fewer people are exploiting that avenue of action. There is then the afore-described compensation in the form of “competitive release” to recompense the cost of deviation from the normative response. Moving from analogy to personality, evolution has selected for variation, even extreme variation of uncommon types which occupy the tail ends of the distribution. The rarity and extremity of these types are the necessary corollary of competitive release, not an indication of dysfunction.  Using any objective biological standard, such as reproductive rate, one will theoretically find, as did Gutiérrez et al. (2013), that traits and persons that are rare extremes will have proportionally similar success when compared to those approximating the optimal response distribution…and so it is that the ideal type, even were it specified in such a way as to correspond with the optimal response distribution, would be subject to amendment through time, and change across space, given then tenets of adaptive diversification. At the same time any ideal type would implicitly and unavoidably, though arbitrarily, devalue competitive alternatives. This fact no doubt will come to the fore as a source of great contention as evolutionary personality psychology is integrated with psychiatric classification, affecting the personality disorders section of the Diagnostic and Statistical Manual of Mental Disorders acutely. In fact, this debate is just augmenting (Leising, Rogers & Ostner, 2009), already showing that some will argue for the continued diagnosis of evolved extremes even in the absence of any objective dysfunctional process (Del Giudice, 2014), while others will argue for the exclusion of evolved extremes from psychiatric nosologies (Wakefield, 1992; Kennair, 2014). Whichever position is taken, subjective diagnosis or no diagnosis, the consolidation of the evolutionary position on personality at this juncture, which shows that “balancing selection leads to two or more different phenotypes (or a continuum of phenotypes) with identical average fitness across environments” (Penke, Denissen, & Miller, 2007), implies that objective diagnosis of personality extremes, based on their deviation from an ideal, will soon become an increasingly indefensible position.



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1) Overdominance is essentially the maintenance of two types because a hybridized version of each is more fit than either of the original contributing types themselves.  The archetypal example of overdominance is sickle cell anemia: Essentially, in malarial regions, it is most preferable to be a carrier for sickle cell anemia; being a carrier would make one heterozygous for the trait.  Being a carrier offers resistance to malaria without exacting the costs of sickle cell anemia, thereby rendering heterozygosity most adaptive in environments with malarial pressures.  However, Penke et al. (2007) conclude: “… it is now widely believed that overdominance is evolutionarily unstable and thus an unlikely candidate for maintaining genetic variation, especially in the long-term.” It has been said that sickle cell anemia is so often used as an example, not simply because it so neatly illustrates overdominance, but also because few other unequivocal examples can be found.


2) Second, antagonistic pleiotropy, as its name suggests, is when “polymorphisms have a positive effect on one fitness-related trait and a negative effect on another (Penke, Denissen & Miller, 2007).” One single trait then, such as coloring, might be buffeted between one of two forms, rendering dynamic stability. Bright color might attract mates, but simultaneously attract predators, such that both bright and muted alleles are balanced within a population, one being more prolific, but with higher attrition rates and the other being more modestly fecund, but with lower attrition rates.  Antagonistic pleiotropy as a mechanism for maintaining trait variation, however, is considered by Penke et al. to be implausible because it is rare that an exact balance will be struck between selective pressures: “Since selection will usually fix the polymorphism with the least total fitness cost, antagonistic pleiotropy could only maintain genetic variation if the fitness costs of all alleles at such a locus are exactly equal…” (Penke, Denissen & Miller, 2007). With some caveats Penke et al. conclude that “Due to these highly restrictive conditions, it is very unlikely that antagonistic pleiotropy plays a major role in maintaining genetic variation.”


3) Frequency dependent balancing selection comes in the negative form discussed, but also in a positive form. Positive frequency dependent balancing selection, instead of promoting diversification, has a homogenizing tendency. Specifically, as one type or trait becomes common it becomes more successful and even more common. If personality were regulated by positive frequency dependent balancing selection, we would have uniformity instead of diversity.





Species and/or genus

Common Name

Intraspecific Variation



Lepomis macrochirus

Bluegill Sunfish

The bluegill sunfish exhibits three morphs based on three male mating strategies: A “parental” strategy that defends the nest, a “sneak” strategy that matures to only a small body size, and a “mimic” strategy that resembles the female form.

Buss & Greiling (1999)


Oncorhynchus kisutch


Some male salmon develop large bodies and fight over territory, while others maintain small bodies and develop large testes allowing them to clandestinely wait for the female to lay a clutch of eggs, which they can then rapidly fertilize with a shower of sperm.

Kenrick et al. (2002);

Geary (2003)


Paracerceis sculpta

Small Marine Isopods 

Among the small marine isopods, “females are mated by large fighter males, intermediate-sized males that mimic females, or small sneaker males.”  These three phenotypes are simply “three alleles at a single autosomal locus,” which are hypothesized to be equally successful at reproduction.

Gross (1996)


Xiphiphorus nigrensis


Another animal with this tripartite variation in males is the “swordtail,” which exhibits “small, intermediate sized, and large males that respectively sneak, sneak and court, and court, females”.

Gross (1996)


Perissodus microlepis

Cichlid Scale-Eating Fish

There are several species of cichlid scale-eating fish present in Lake Tanganyika. These fish swim rapidly up behind another fish and bite off a swath of scales.  As the scale-eating fish passes the prey fish rapidly and alongside, it helps greatly if their mouthparts are angled.  And so evolution has designed them.  What is interesting is that some have mouthparts angled to the right and some to the left.  There are roughly equal numbers of right and left because as one becomes more common the other has an advantage.  If most struck from the left, prey fish would observe and deflect more attacks from the left.  Consequently, those rare fish that struck from the right would have an advantage and be more successful scale eaters.

Stearns & Hoekstra (2005)




The damselfly shows two male morphs, one sneaking with clear wings and another one traditionally territorial and aggressive.  The former live longer, the latter are more reproductively successful over their short lives, both equal out in terms of fitness

Sherrat & Wilkinson (2009)




There are two female morphs in dragonflies; one that is traditionally female and one that is a male mimic, gaining fitness advantage by avoiding costly male sexual harassment.

Sinervo & Calsbeek (2010)





Color patterns vary in certain species of male guppies so that populations are distributed across three types.  Experimentally reducing the frequency of any one type confers a slight fitness advantage on that type.

Olendorf  et al. (2006)


Loxia curvirostra


Crossbills have beaks that overlap in one of two directions, and populations seem to be balanced between these types.  This seems to be regulated by frequency dependent selection; as one type can access only parts of the cones which they feed off.  What one can exploit the other cannot and so both types are represented, such that at any given time there should be a balance, just as there usually is within animal sex ratios, which is also regulated by negative frequency dependent balancing selection. 

Benkman (1996)


Erythrura gouldiae

Gouldian Finches

Gouldian finches have two male types; one a highly competitive red headed finch that suffers much social stress and has high, sustained cortisol levels and a black headed finch that is less competitive and consequently less physiologically taxed by adrenal stress.

Pryke et al. (2007)


Pongo pygmaeus


Among orangutans, “adult males can appear in two morphs: a large morph with the full accompaniment of the male sexually dimorphic traits of fleshy cheek pads and throat sac. And a small morph that lacks adult male phenotypic soft-tissue traits. The smaller morph appears developmentally arrested in a permanent adolescent state, but is actually a fully reproductive adult. Males distinguished by this morph pursue an alternative reproductive strategy, but also enjoy the benefit of needing less food to support their smaller body size…”.

Cachel (2006; page 234)


Salvelinus alpinus

Arctic Char

“In Arctic char, four sympatric morphs reflecting dietary or habitat differences can actually occur within the same lake. To a non-specialist, the differences between the morphs equal those found between different freshwater fish species”.

Cachel (2006; page 218)


Authors’ information: 

Steven C. Hertler

Adjunct-Assistant Professor

Department of Psychology

College of New Rochelle

New Rochelle, New York 10805

E-mail: stevenhertler@hotmail.com








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