It is now a “rule” that “all human behavioural traits are heritable” (Turkeimer, 2000. cited in Pinker 2002). Pinker says that when psychologist Eric Turkheimer made this bold claim in 2000, he was encapsulating over 40 years of studies that overwhelmingly and robustly support this view, and only slightly exaggerating (Pinker 2002). Estimates of the mean heritability for the fundamental traits in the most popular model of human personality (the Five Factor Model) are estimated to be between 42% (for agreeableness) and 57% (for openness to experience) (Bouchard & McGue, 2003).
The obvious questions arise from these important and robust findings – is natural selection involved in moulding personality and, if so, how. This paper will discuss three main models of evolutionary selection relating to personality that have been proposed to answer these questions. Most of the work is speculative and observational generalities rather than experimentally derived findings are generally used, so any comparison of the theories is mostly based on the internal logic of the arguments and their concordance with current understanding of more fundamental genetic research.
For the purpose of this paper, the terms behavioural traits and personality traits, although not identical, are used interchangeably. When considering a role for natural selection in developing personality traits the problem that arises is how to account for the wide variation in expression of these traits, where only the most extreme presentations appear to be disadvantageous and thus rare (Buss, 2008; Nettle, 2006; Penke, Denissen & Miller, 2007). For example, the trait of openness to experience has a very wide range of expression, from astronauts to the contemplative monk and everything in between.
Attempts to understand the existence of such large individual variation within personality has led to three main proposed explanations, which we will call selective neutrality, mutation-selection balance and balancing selection, using the terminology employed by Penke, Denissen and Miller (2007). Each of these proposed models is supported by sound evidence and logical arguments, and will be considered in turn. Selective neutrality. Tooby and Cosmides (1990) suggest that variations within personality traits are simply genetic noise because they are selectively neutral.
They state that “…one should not expect there to be any important variation in traits that have a history of selection”. This is based on the assumption that natural selection must always lead to a single, species wide optimal adaptation that leads to maximum reproductive fitness. However, Penke, Denissen and Miller (2007) argue that while this is logically possible, it is unlikely for three reasons. Firstly, using mathematical models of evolutionary forces, they show that in the long term, genetic drift is the only force that can act on selectively neutral mutations, and that genetic drift is always towards decreased genetic variance.
Secondly, they argue that, again for mathematical reasons, it becomes less likely for traits to be invisible to selective forces as the population involved increases. Thirdly, they suggest that as personality traits affect such a wide range of behaviours that are themselves known to influence genetic fitness, such as finding a mate, it is extremely unlikely that they could remain selectively neutral. Mutation-selection balance.
Bouchard and Loehlin (2001) suggest personality traits are continually attempting to achieve a balance between new mutations arising and extreme variations (both positive and negative) being minimized or eliminated. Penke, Denissen and Miller (2007) show that when traits that are not selectively neutral are affected by a large number of mutations across a large number of genetic loci, they lead to this type of balance, rather than lead to a single optimal trait. Since humans have a relatively high mutation rate and personality traits are affected by a very large number of genetic loci, this type of balance is quite possible.
However they suggest that the more generalized (or fundamental) traits that reflect the general mutation load most accurately, such as good looks, physical symmetry, or general intelligence, are those most likely to achieve mutation-selection balance. Balancing Selection. The third option, balancing selection, suggests that a number of different expressions of a trait may be selected because the fitness of each of them is, on average, equal due to the environments they exist in varying either
over time or location (Buss, 2009; Penke, Denisson & Miller, 2007). For example, an environment that fluctuates between plenty and famine will tend to favour both high risk takers, who will search harder and further for food during famines, as well as low risk takers, whose cautious behaviours keep them safer as long as food is plentiful. Averaged over time, people exhibiting each type of behaviour have approximately equal reproductive success.
Balancing selection includes a sub-type called frequency-dependent selection, when a number of different expressions of a trait may coexist if they each achieve maximum fitness when existing as a specific proportion of the population (Buss, 2009; Sterelny & Griffiths, 1999). Penke, Denisson and Miller (2007) state that this generally occurs when the changes in selection pressures are within the social rather than physical environment. This suggests that frequency-dependent selection is particularly relevant to human personality traits, insofar as they are closely connected to the social environment.
For example, Buss (2008) suggests that psychopathy, with its callous disregard for others and group norms, may be a successful strategy for an individual providing it exists only in very small numbers in any population. Should the numbers rise, awareness of their negative behaviours would lead to sanctions and it would become a less successful strategy, and numbers would eventually decrease (either through gaol in the short term or failure to reproduce because of social sanctions).
The endpoint of balancing selection is that each phenotype reaches its maximum fitness level when existing as the right proportion of the population resulting in an evolutionary stable strategy (ESS) (Penke, Denisson & Miller, 2007). Nettle (2006) takes this further and suggests that the five major personality traits each have such a broad range of possible expression because the varying levels of each trait have both positive and negative adaptive effects, and because no trait is adaptively successful when taken to extremes.
This, combined with the complexities of the human social environment (the most relevant to personality), could allow many niches or different environments for a large variety of behaviours to be equally successful. For example people high on the neuroticism trait may be more successful than others in a more dangerous environment or period of time, but when the danger decreases, they may be inhibited by anxiety from taking advantage of improved conditions and thereby will be less successful.
However, their adaptive success may be equal (on average) to others with much less neuroticism who were less successful in the first environment but more so in the second. While this idea is still only speculative, it is hard to envisage how this theory could be investigated and Nettle himself does not have concrete suggestions. However, Buss (2008) reports an interesting study by Ciani, Capiluppi, Veronese and Cartori that provides circumstantial support for Nettles’ theory that different environments can lead to a different balance of personality traits.
Two different groups of people, one living in mainland Italy, the other living on small islands off the Italian coast, were assessed for the Big Five personality traits. Those who had lived on the islands for more than 20 generations were found to have significantly lower scores on the extraversion and openness to experience traits than those on the mainland as well as those who had moved to the islands more recently. While a difficult study to reproduce, this may suggest avenues for future research.
In conclusion, we have discussed three main responses to the question of whether the heritability of personality traits means they are affected by evolutionary selection? The first, selective neutrality, suggests that variations within personality traits are genetic noise because they make no difference to evolutionary fitness. However this seems very unlikely, since the major personality traits correlate with behaviours directly relevant to survival, such as mating, socializing and aggression.
Also selection neutrality becomes much less likely to occur as populations increase significantly as the human population has done. The second, mutation-selection balance, is that selection occurs, but is in a constant state of flux between new mutations being created and less successful mutations being selected out. While this is a plausible scenario, it is more likely that this occurs in traits that are so broad ranging in their effect that they act as general indicators of fitness, such as general intelligence.
The final possibility, balancing selection, where more than one expression of each trait is selected for due to either variety or change in the environment seems the most likely. The human social environment is particularly varied and changing, and this huge diversity in the social landscape makes it very likely that many different behavioural strategies could become equally adaptively successful or find its own small niche. Perhaps the most likely answer is a combination of all three of these solutions, as suggested by Penke, Denissen & Miller (2007).
In this view, smaller or less important personality traits, such as love of music, may be selection neutral, the very fundamental or wide ranging traits, such as intelligence, may be in a state of mutation selection balance due to the very large number of genetic loci involved, while the more important personality traits, may undergo balancing selection. This is an area of study still in its infancy. References Bouchard, T. J. Jr. & Loehlin, J. C. (2001). Genes, evolution and personality. Behavior Genetics, 31, 243-273. Bouchard, T. J. Jr. & McGue, M. , 2003.
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