1. Presentation by: Nikoo Naimi and Sarah Krouse
Nettle, D. (2006). The evolution of personality variation in humans and other animals. American Psychologist, 61 (6),
Presentation by: Nikoo Naimi and Sarah Krouse

2. Research Basis
Previous research hasn’t paid much attention to variation in traits because they believed, due to natural selection, important traits would not have much variation.
Tooby and Cosmides (1992), Buss (1991), and Buss and Greiling (1999) suggest, however, that there may be some heritable variations that serve a functional purpose.
Nettle, D. (2006). The evolution of personality variation in humans and other animals. American Psychologist, 61 (6),

3. Variation through Trade-Offs
It is suggested that variation can come about through trade-offs. Where individuals within the same normal range of a personality dimension may achieve the same levels of fitness through different means.
This means that some tradeoff exists, where if one component of fitness is maximized by increasing a trait, another component will suffer.
Nettle, D. (2006). The evolution of personality variation in humans and other animals. American Psychologist, 61 (6),

4. Purpose of Article
To review variation between individuals in nonhuman species, especially in functionally relevant traits.
To review how a different traits in the five-factor model of personality can be seen as a cost or benefit depending on the situation.
Nettle, D. (2006). The evolution of personality variation in humans and other animals. American Psychologist, 61 (6),

5. Variation in Non-human Species
Especially in functionally relevant traits
Example:
Within the pygmy swordtail, large males are desired but take longer to develop. Smaller males develop sooner and are able to sneak up on the women and copulate with them. Therefore, there is a trade-off of costs and benefits to each variation. Different contexts yield different advantages to each variation.
Nettle, D. (2006). The evolution of personality variation in humans and other animals.
American Psychologist, 61 (6),

6. Five Factor Model of Personality: Predicted Benefits and Costs
Personality Traits
Benefits
Costs
Openness
Creativity, which leads to mating success
Leads to paranormal beliefs and psychosis
Conscientiousness
Longer life expectancy; attention to long term goals
Overlooking spontaneous reproduction possibilities; associated with eating disorder and obsessive compulsive disorder
Extraversion
Have more mates; explore their environment
Expose self to risk; lack of family stability
Agreeableness
Aware of others’ mental states; avoid violence; cooperative; valued friend and coalition partner
Excessive trusting in others; missing out on opportunities
Neuroticism
Avoiding dangerous situations; detecting and quickly reacting to threats; motivating competitiveness
Psychiatric disorders (i.e. depression and stress); damages to physical health
Adapted from Nettle, 2006, p.628
Nettle, D. (2006). The evolution of personality variation in humans and other animals. American Psychologist, 61 (6),

7. Conclusions
There is considerable variation in each trait, and depending on the context and situation, each trait may be seen as beneficial or harmful.
Evolution results in considerable variation within animal and human populations. The notion of trade-offs has been used to explain diversity, because, due to selection, fluctuations in the environment lead to variation in phenotypes.
Nettle, D. (2006). The evolution of personality variation in humans and other animals. American Psychologist, 61 (6),

8. Critical Review Comments
Interesting Points
Fluctuations in the environment lead to heritable variation (e.g. Galapagos finches).
In nonhuman species, fitness functions are not necessarily unimodal (e.g. two male morphs in coho salmon).
Openness is associated with depression and schizophrenia.
Points of Weakness
Trade-offs and fluctuating selection are not the only possible explanations for heritable variation.
No experimental design to test hypothetical costs and benefits of trait variation.
Does not provide readers with definitions of agreeableness and openness.
Nettle, D. (2006). The evolution of personality variation in humans and other animals. American Psychologist, 61 (6),