Truism of the day: Introverted nerds don't get laid much. Or, in more scientific terms, extroversion in men is linked to a higher number of sex partners.[1] Men and women have similar levels of extroversion, though-- in fact, women have slightly higher levels of it.[2] This begs for explanation; after all, extroversion is significantly heritable (~50%)[3], so why shouldn't it have been positively selected for in males?

It turns out that, while men aren't more extroverted than women, they are more extroverted in the areas where it "counts.""The table below[2] of extroversion and its sub-traits sheds some light:

Trait	Mean Difference (Female - Male)	High (%F)	Very Low (%F)
Extraversion	1.8	58	44
Warmth	1.3	67	35
Gregariousness	1.0	63	38
Assertiveness	-0.9	45	58
Activity	0.5	64	45
Sensation seeking	-1.5	42	70
Positive emotions	1.3	78	36

(mean difference is the mean difference between men and women on the trait. high %F and low %F are the percent of women who are at the very high and very low tail-ends of the distribution.)

With the exception of 'ideas' (F-M=-1.6), a sub-trait of Openness to Experience, none of the 30 Big-Five sub-traits show more skew towards men than do assertiveness and excitement-seeking. While these -.9 and -1.5 mean differences may seem minor on their face, it is worth considering how they affect the tails of the distributions. In the case of sensation-seeking, 70% of people who are significantly low on excitement-seeking are female. A great deal of meaningful sexual dimorphism here, so let's look into it...

Sensation-seeking

Browsing through the sensation-seeking literature I came across this very interesting study; sensation-seeking was one of many variables examined in a study of college mens' number of sexual partners.[4] The other variables looked at (and all measured through questionnaires unless otherwise indicated) were: age, attractiveness (measured by self-rating, and female, male interviewer ratings), social intimacy, sexual affect, dominance, hypermasculinity, Eysenck's psychoticism trait measure, and testosterone levels (measured chemically through saliva samples).

Sensation seeking correlated more so than any other variable with both lifetime number of sexual partners (.38) and with maximum partners in one month (.37). Trailing way behind it was hypermasculinity (.29, .29), followed by attractiveness (.20, .28).[4,5]

I'm not going to attempt to unwind the complex causal chains which correlate sensation-seeking to short term mating success. It should suffice to say that there is significant evidence that these traits are both intrinsically attractive to women, and that they serve as an impetus to sexual pursuit of women by men in the first place.[7] Sensation- seeking has the highest narrow-sense heritability of any (Big-5) sub-trait-- .36, and a relatively high broad-sense heritability of .52, by the way.[3]

A Pet Hypothesis

It seems plausible that sensation-seeking garnered a greater number of female mates in the Pleistocene , just as it does now, and that there was therefore positive selection for it in men. I would posit that if this positive selection existed, it was limited in effect by the negative aspects of extroversion, visible in our day in age-- extroverts are more at risk for STD's, being jailed, getting in fights, and generally doing stupid risky things.

The fact that Extraversion has a higher degree of heritability in men than in women (.57 versus .38)[3] might be considered as evidence. I am not knowledgeable enough of the behavior genetics involved to say whether this is meaningful evidence.

The most specific, and testable part of my hypothesis is this: that ADHD (note that I'm not saying ADD) is to some extent the result of "overclocking" for male sensation-seeking. Consider this-- estimates of the male:female ratio for ADHD range from 4:1 to 9:1.[8] People with ADHD are more extroverted than other people, yes, but they are especially more sensation-seeking than other people. Unsurprisingly, people with ADHD have a higher number of sexual partners than people without.[9] The discrepancy between males without ADHD and those with it is probably underestimated because of the widespread use of drugs like Ritalin.

Notes

1. (Nettle 2004).
2. (Corbitt & Widiger 1995). See the table in their article for all of mean differences and percentile differences at the tails of the bell curves between men and women.
3. (Loehlin & Bouchard, 2001)
4. (Bogaert et. al 1995)
5. Sensation seeking correlated trivially with age, .14 with Attractiveness, .26 with dominance, .41 with hypermasculinity, and .45 with psychoticism. Statistically eliminating virgins from the sample had no major effects on these correlations. For you data crunchers out there I suggest you read the study yourself if you want to analyze their factor and regression analyses.
6. (McCoul & Haslam 2001).
7. There's a good summary of some of the studies on why sensation-seeking might cause more mates in (McCoul & Haslam 2001).
8. (Gutman 2002)
9. (White 1998)

References

• Nettle, D. (2004). An evolutionary approach to the extraversion continuum. Evolution and Human Behavior
• Corbitt, E.M. and W.A. (1995). Sex Differences Among the Personality Disorders: An Exploration of the Data. Clinical Psychology: Science and Practice.
• Loehlin, J.C., & Bouchard, T.J. (2001). Genes, Evolution, and Personality. Behavior Genetics.
• Bogaert, A.F. and Fisher, W.A. (1995). Predictors of University Men's Number of Sexual Partners. The Journal of Sex Research
• Maryann D. McCoul and Nick Haslam (2001). Predicting high risk sexual behaviour in heterosexual and homosexual men: the roles of impulsivity and sensation seeking. Personality and Individual Differences
• Gutman, A. (2002). ADHD -- Perspectives From Child to Adult. Retrieved July 18, 2008, from the MedScape Web site
• White, J.D. (1998). Personality, temperament and ADHD: a review of the literature. Personality and Individual Differences.
• Kate et al (2006). Childhood ADHD Predicts Risky Sexual Behavior in Young Adulthood. Journal of Clinical Child and Adolescent Psychology

Labels: Personality, sex, sex differences