Can Evolutionary Psychology Explain Individual Differences in Personality?

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“Yes, we are all individuals!”
Can evolutionary psychology explain individual differences in personality?

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...
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