Wednesday, September 02, 2009

Where the Sexy Ones Are   posted by Razib @ 9/02/2009 01:04:00 PM

Tyler Cowen Alex Tabarok points me to this paper, Sociosexuality from Argentina to Zimbabwe. The abstract:
The Sociosexual Orientation Inventory (SOI; Simpson & Gangestad 1991) is a self-report measure of individual differences in human mating strategies. Low SOI scores signify that a person is sociosexually restricted, or follows a more monogamous mating strategy. High SOI scores indicate that an individual is unrestricted, or has a more promiscuous mating strategy. As part of the International Sexuality Description Project (ISDP), the SOI was translated from English into 25 additional languages and administered to a total sample of 14,059 people across 48 nations. Responses to the SOI were used to address four main issues. First, the psychometric properties of the SOI were examined in cross-cultural perspective. The SOI possessed adequate reliability and validity both within and across a diverse range of modern cultures. Second, theories concerning the systematic distribution of sociosexuality across cultures were evaluated. Both operational sex ratios and reproductively demanding environments related in evolutionary-predicted ways to national levels of sociosexuality. Third, sex differences in sociosexuality were generally large and demonstrated cross-cultural universality across the 48 nations of the ISDP, confirming several evolutionary theories of human mating. Fourth, sex differences in sociosexuality were significantly larger when reproductive environments were demanding but were reduced to more moderate levels in cultures with more political and economic gender equality. Implications for evolutionary and social role theories of human sexuality are discussed.

Below is a table of SOIs....


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Friday, July 18, 2008

sextroverts   posted by ben g @ 7/18/2008 01:20:00 PM

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:

TraitMean Difference (Female - Male)High (%F)Very Low (%F)
Sensation seeking-1.54270
Positive emotions1.37836

(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...


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.


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)

  • 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

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Monday, June 30, 2008

Increased rates of sexually transmitted diseases amongst the older   posted by Razib @ 6/30/2008 01:55:00 AM

Doubling Of Sexually Transmitted Infections Among Over-45s In Under A Decade. Dare we say an "epidemic???" If you want to push the envelope of course, She was 82. He was 95. They had dementia. They fell in love. And then they started having sex. In any case:
While the numbers of infections identified in younger age groups rose 97% during the period of the study, those identified in the over 45s rose 127%.
"Indeed, it may be argued that older people are more susceptible [to sexually transmitted infections] as they are less likely to use condoms than younger people," they say, adding that as successive waves of people with more liberal sexual attitudes and behaviours age, the problem is likely to worsen.

I guess the "safe sex" message just isn't getting through to the less young.

Related: Your generation was sluttier.

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Friday, April 27, 2007

Intercourse and Intelligence   posted by Jason Malloy @ 4/27/2007 05:19:00 PM

Tyler Cowen quotes from a new study testing the relationship between grades and delayed sexual activity.

Last December I passed a paper along to Razib showing that high-school age adolescents with higher IQs and extremely low IQs were less likely to have had first intercourse than those with average to below average intelligence. (i.e. for males with IQs under 70, 63.3% were still virgins, for those with IQs between 70-90 only 50.2% were virgin, 58.6% were virgins with IQs between 90-110, and 70.3% with IQs over 110 were virgins)

In fact, a more detailed study from 2000 is devoted strictly to this topic, and finds the same thing: Smart Teens Don't Have Sex (or Kiss Much Either).

The team looked at 1000s of representative teens grades 7-12 in the National Longitudinal Study of Adolescent Health and The Biosocial Factors in Adolescent Development datasets, both of which include an IQ test, and include detailed sexual experience questions ranging from hand-holding to intercourse. As with the other study there was a curvilinear relationship: students with IQs above 100 and below 70 were significantly less likely to have had intercourse than those in between. Also like the other study, they found teens with IQs ranging from 75 to 90 had the lowest probability of virginity (the authors note this is also the same IQ range where propensity towards crime peaks).

Depending on the specific age and gender, an adolescent with an IQ of 100 was 1.5 to 5 times more likely to have had intercourse than a teen with a score of 120 or 130. Each additional point of IQ increased the odds of virginity by 2.7% for males and 1.7% for females. But higher IQ had a similar relationship across the entire range of romantic/sexual interactions, decreasing the odds that teens had ever kissed or even held hands with a member of the opposite sex at each age.

While these authors leave off at grade 12th, it would seem plausible to expect that this relationship extends beyond high school. To explore this, plenty of interesting facts come from a 2001 campus sex survey by the joint MIT/Wellesley college magazine Counterpoint (PDF). Looking within and between colleges, IQ appears to delay sexual activity on into young adulthood.

By the age of 19, 80% of US males and 75% of women have lost their virginity, and 87% of college students have had sex. But this number appears to be much lower at elite (i.e. more intelligent) colleges. According to the article, only 56% of Princeton undergraduates have had intercourse. At Harvard 59% of the undergraduates are non-virgins, and at MIT, only a slight majority, 51%, have had intercourse. Further, only 65% of MIT graduate students have had sex.

The student surveys at MIT and Wellesley also compared virginity by academic major. The chart for Wellesley displayed below shows that 0% of studio art majors were virgins, but 72% of biology majors were virgins, and 83% of biochem and math majors were virgins! Similarly, at MIT 20% of 'humanities' majors were virgins, but 73% of biology majors. (Apparently those most likely to read Darwin are also the least Darwinian!)

Looking at this chart it would strongly appear that higher complexity majors contain more virgins than majors with lower cognitive demand. This paper provides me with GRE scores by academic discipline, and, in fact, the correlation between the percentage of virgins in each Wellesley major and the average 'Analytical' GRE score associated with the discipline is 0.60.

One reason we might guess that smarter people in high school, or in more challenging colleges or majors, delay their sexual debuts is because they are delaying gratification in expectation of future reward. Sexual behavior (or at least the investment needed to procure a partner or sustain one) may compete with time/resources required for other goals, and intelligent people may have more demanding goals. James Watson even hinted at this in a recent Esquire magazine piece:

If I had been married earlier in life, I wouldn't have seen the double helix. I would have been taking care of the kids on Saturday. On the other hand, I was lonely a lot of the time.

While sex may not be marriage, it may still require effort that intelligent people prefer to invest elsewhere. This would fit Aldus Huxley's alleged definition of an intellectual as a person who's found one thing that's more interesting than sex.

Another idea is that smarter people are more risk averse, and delaying these activities is a byproduct of enhanced concerns about unwanted pregnancy and disease. While not avoiding sexual behaviors, per se, they are just less likely to seek it out or consent to it for fear of the potential consequences.

Another idea is that smarter people are more religious or more ethically conservative, and are trying harder to wait for marriage to have sex.

Another idea, consistent with popular media portrayals of geeks and nerds (males at least), is that intelligent people actually want to have sex, but are simply less likely or unable to obtain willing partners because they are disproportionately viewed as unattractive or undesirable as partners.

Another idea is that intelligent people have lower general sex drives. This shouldn't be confused with the first theory, where their sex drives would be normal and they have greater self-restraint.

Some insightful digging by blogger Half Sigma into the General Social Survey, which also includes an abbreviated intelligence test, has turned up a number of associations that speak to these theories. The relationship between sexual activity and intelligence found across adolescence and young adulthood appears to continue on into adulthood proper.

Not only do intelligent people have a delayed onset of sexual behavior, Half Sigma found that they also have a lower number of premarital sex partners throughout adulthood (18-39). While this is consistent with the above theory that high IQ people are more religious and conservative, this is, of course, not true. Religiousness correlates with lower IQ, and as HS shows in the same post, intelligent people were also more likely to say that premarital sex was not immoral. (Leaving those who did think it was immoral to participate in the bulk of it!) Most of the other theories are still consistent with this finding though.

Perhaps more revealing, HS, also showed that intelligence correlates with less sex within marriage for the same age range. While still consistent with pregnancy fears and competing interests, lower sex drive seems like a better fit. In fact another revealing finding from the Counterpoint survey was that while 95% of US men and 70% of women masturbate, this number is only 68% of men and 20% of women at MIT!

Also the idea that more intelligent people are too busy for the opposite sex not just in 7th grade to college, but throughout adulthood and for their own spouse, seems unrealistic. In fact the GSS also shows (PDF) that smarter people spend more time socializing with their friends, indicating their hours aren't spent as uniquely isolated and narrowly channeled as the theory would require.

But lower sex drive and anxiety about sex's consequences can't be the whole story either. Half Sigma also showed that the smartest men in the GSS (approx. IQ >120) were also more likely to visit a prostitute. (Hardly indicative of cautiousness) This may suggest intelligent men are less able to find willing sex partners. Are smart men less attractive to women? Perhaps in some ways. For instance HS found that smart men were less likely to be athletic, and this paper shows, unathletic men and women have fewer sex partners. Athletic men, with more willing sexual partners are also less likely to visit a prostitute. Athletic activity gives men more masculine bodies, which are more attractive to women. A more masculine physique correlates with (PDF) an increased number of sex partners.

So intelligent people have lower libidos and less masculine physiques. What hormone is responsible for both sex drive and masculine builds? That's right: testosterone.

And two new papers suggest that testosterone may depress IQ. One team found that salivary testosterone levels were lower for preadolescent boys with IQs above 130 and below 70. (the same two groups most likely to be virgins in adolescence)

Another paper suggests that a gene responsible for androgen sensitivity and higher sperm counts may also create a tradeoff for intelligence.

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Tuesday, February 20, 2007

Left-handedness and pedophilia: Brain damage   posted by agnostic @ 2/20/2007 05:58:00 PM

It seems that "pathological" left-handedness and pedophilia might share a common origin in some early developmental disturbance(s), possibly a brain infection (or group of infections). I first read about the potential fitness costs that left-handers suffer in a passing remark of Harpending & Cochran (2006), although when I searched PubMed for "left-handed" and "longevity," I found that the results have been somewhat inconsistent. However, a new study in Epidemiology on longevity and handedness in a large, representative sample of Dutch women (Ramadhani et al. 2007) confirms the earlier reports of decreased longevity among left-handers. The Hazard Ratios for left-handers vs. right-handers showed that lefties were more likely to die of all causes, and even more so for specific causes such as colorectal cancer and cerebrovascular diseases (PDF pp. 2-3):

Table 2 shows that, after adjustment for age, SES, BMI, and cigarette smoking status, left-handed women had a 1.36 times higher risk of dying from all causes than non-left-handed women. The adjusted HR for total mortality, after excluding the first 5 years of follow-up time, was 1.58 (95% CI = 1.03--2.42). With regard to cancer mortality, left-handed women had a 1.7 times greater risk of dying from any type of cancer (CI = 1.0--2.7), a 4.6 times higher risk of dying from colorectal cancer (1.5--14), and a 2.0-fold higher risk of dying from breast cancer (0.83--4.6). Handedness was weakly associated with overall mortality from diseases of the circulatory system (1.3, 0.54--3.3), although left-handed women had a 3.7 times greater risk of dying from cerebrovascular diseases than non-left-handed women. Left-handedness was not associated with mortality attributable to causes other than the above-mentioned.

A separate study from the same group of researchers (Ramadhani et al. 2006) also found that the severity of a bacterial meningitis infection correlated with handedness, such that those with stronger infections were more likely to become left-handed and lower in IQ. They note that since the infection happens before the other events, the former likely contributes to causing the latter (pp. 2528-9):

Fig. 1 shows that children with a meningitis severity score above the median had a 6.2 times higher risk of becoming lefthanded at school age compared to those below the median (95% CI 2.0--18.6). Furthermore, those who contracted meningitis below the median age of 1.8 years had a 12.3 times higher risk (95% CI 2.6--58.0) compared to a 5.9 times higher risk (95% CI 1.6--21.7) among children who contracted meningitis at older age.
. . .
More specific analyses on cognitive function are shown in Table 3, with left-handed children generally performing worse on the cognitive tests. Left-handed children had an almost seven points lower IQ (p = 0.018), a one point lower vocabulary score of WISC-r (p = 0.061), and an almost five points lower Beery score (p = 0.069) than their non-left-handed counterparts.

Turning next to handedness among pedophiles, Bogaert (2001) examined a Kinsey Institute dataset for the handedness data of 286 criminal pedophiles (those who had a victim of either sex under age 12). His findings, where NRH means Non-Right-Handedness (pp. 467-8):

Although the effects were small, it should be noted that a 5% difference means about a 30% change in NRH (e.g. 15.7% in pedophiles vs. 11.5% in controls).
. . .
In contrast, education is not related to the handedness/pedophilia relationship. Pedophiles had elevated NRH relative to controls with or without controlling for education. This result suggests that, even though pedophiles have a rate of NRH comparable to other offenders (or perhaps slightly higher), there may be a different mechanism underlying the handedness/pedophile relationship than the handedness/(general) criminality relationship. Thus, elevated NRH in pedophiles probably does not merely represent a 'criminal/antisocial behavior' tendency, because of, for example, general cognitive and/or educational difficulties. Instead, these data may indicate that elevated NRH in pedophiles reflects [Central Nervous System] abnormalities that, in part, directly affects their sexual preference systems (see [24] for a review of CNS abnormalities in pedophiles).

Next are two studies done by Cantor and colleagues. In the first, Cantor et al. (2004) examined men who were referred to a clinic for inappropriate sexual behavior, using phallometric responses (which measure penile arousal) to various erotic stimuli in order to divide the men categorically into pedophiles (those attracted to children under 12), hebephiles (those attracted to 12-16 year-olds), and teleiophiles (those attracted to adults). They found (p. 8):

The group differences did not change appreciably after the addition of the covariates [i.e., Age and Wechsler Full Scale IQ; agnostic], F(2, 294) = 6.31, eta^2 = .041, p = .002, and simple effects contrasts showed that both the pedophilia group and the hebephilia group reported significantly less right-handedness than did the teleiophilia group, t(294) = -3.51, eta^2 = .040, p = .001 (twotailed), and t(294) = -2.14, eta^2 = .015, p = .033 (two-tailed), respectively.

The researchers then used a continuous measure of sexual interest, namely the degree to which they become aroused when viewing different stimuli (p. 8-9):

The results indicated that, in general, measures of cognitive ability correlated negatively with sexual response to children and positively with sexual response to adults. Handedness, however, was significantly related to phallometric response to children, only. [From Table 4, partial correlation of arousal by prepubescent stimuli and handedness = -.13, p less than .05, where negative handedness indicates left-handedness; agnostic.] There was little evidence of association between the neuropsychological variables and sexual response to stimuli depicting pubescents in the laboratory.

They conclude that the relationship between handedness, cognitive ability, and pedophilia is that they are all caused by an early collection of brain perturbations, not that poor cognitive ability leads one (probabilistically) to develop pedophilic interests, since IQ was controlled for. Their follow-up article (Cantor et al. 2005) has two studies, the first of which was a successful replication of their previous findings. The second study classified patients categorically as right-handed or non-right-handed, in contrast to their continuous measure in the 2004 study, so that they could report Odds Ratios, making their results more comparable to those of other studies. By measuring penile arousal to stimuli involving adults, pubescents, and prepubescents, as before, the authors slotted the patients into those primarily attracted to one of the three age groups, as well as which sex they were most attracted to.

The relationship between sexual preferences and handedness was only significant for those attracted to prepubescents (although there was a trend toward lower right-handedness among those attracted most to pubescents), with or without controlling for IQ. Their Table II shows that after controlling for IQ in their logistic regression, the B-coefficient for age preference in predicting handedness was 1.26 (SE = .34), and the Odds Ratio was 3.54 (95% CI = 1.84--6.81), p = 0.0002. Next the researchers categorized the patients based not on their penile response to stimuli, but on their actual sexual offenses against others, again sorting by age and sex of the victim. At first, there was a trend in the same direction as the results obtained from measuring genital response, but the trends were not significant.

They thought that incest cases might be confounding the results -- for example, men who commit crimes against intrafamilial individuals might be engaging in a facultative behavior similar to men who have sex with men in prison, rather than have obligate pedophilic preferences. After removing patients who had intrafamilial victims, the results became significant. In particular, their Table V shows that in a similar regression analysis as before, the best predictor of handedness was a preference for prebubescents, even controlling for IQ: B = 1.06 (SE = .47), OR = 2.9 (95% CI = 1.15--7.28), p = 0.02. Admittedly the results are less striking when measuring sexual preferences according to their actual sexual offenses than when measuring it by their arousal to erotic stimuli, but this may well be because the latter is a purer measure of preferences, since it doesn't involve the vagaries of choosing and offending against a real person.

In sum, there is a significant relationship between non-right-handedness and pedophilic preferences, and this is likely the result of an early developmental disturbance to the brain. Since these studies were conducted in modern Western countries, things like lack of proper nutrition in the mother are probably not what's going on. As hinted at in the study of meningitis and handedness, it is more likely that an infection causes the damage -- these are one of the sources of environmental insults whose effects we still have yet to curtail, beyond ameliorating some of the more horrendous cases like smallpox. Of course, the pathogen responsible for non-right-handedness doesn't have to be the same as that which causes pedophilia; the affected individual might have been born in an unusually high pathogen-load area, or have a weakened immune system in general, and so on.

One thing seems pretty clear, though: common cases of deviance from Darwinian fitness are most likely caused by environmental insults, with pathogens being the most obvious culprit (see Cochran, Ewald, & Cochran 2000 for the rationale). That result isn't guaranteed a priori, but it is a far more reasonable "working hypothesis" for a particular case than imagining how, for example, pedophilia might have been adaptive in the past, or how it represents the tail of a distribution for "caring for children," or how pedophilia-normal heterozygotes might enjoy a fitness advantage, and so on. Mental, financial, and time resources are all limited, so we should follow the course that all good detectives do: first look at the person you most suspect, based on the accumulated knowledge of similar past cases. Real life is not a Law & Order episode where the least likely culprit routinely turns out to be the one who dunnit. If they're cleared, then move on to the exotic suspects.

Related: Left-handedness post from the archives.


Bogaert, AF (2001). Handedness, criminality, and sexual offending. Neuropsychologia, 39, 465-9.

Cantor, JM, R Blanchard, BK Christensen, R Dickey, PE Klassen, AL Beckstead, T Blak, & ME Kuban (2004). Intelligence, memory, and handedness in pedophilia. Neuropsychology, 18, 3-14.

Cantor, JM, PE Klassen, R Dickey, BK Christensen, ME Kuban, T Blak, NS Williams, & R Blanchard (2005). Handedness in pedophilia and hebephilia. Archives of Sexual Behavior, 34, 447-59.

Cochran, GM, PW Ewald, & KD Cochran (2000). Infectious causation of disease: an evolutionary perspective. Perspectives in Biology and Medicine, 43, 406-48.

Harpending, H & G Cochran (2006). Genetic diversity and genetic burden in humans. Infection, Genetics, and Evolution, 6, 154-62.

Ramadhani, MK, SG Elias, PA van Noord,DE Grobbee, PH Peeters, & CS Uiterwaal (2007). Innate handedness and disease-specific mortality in women. Epidemiology, 18, 208-12.

Ramadhani, MK, I Koomen, DE Grobbee, CA van Donselaar, A Marceline van Furth, CS Uiterwaal (2006). Increased occurrence of left-handedness after severe childhood bacterial meningitis: support for the pathological left-handedness hypothesis. Neuropsychologia, 44, 2526-32.

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