Twin studies are a scientist and media favorite. Because twins share the womb, their pre-natal environment is identical. Their upbringing, in most cases, can also be assumed to be similar -- even more so than for non-twin siblings. However, most twin studies focus on identical twins, in order to control for genetic differences as well. But in the study of gender differences, opposite-sex twins (necessarily fraternal) may be even more enlightening.

Procopio and Marriott (2007) found that males with a female twin did not have a different risk of developing anorexia nervosa than their twin sisters. Resnick et al (1993) (in a re-analysis of data collected in the 1970's) demonstrated increased sensation-seeking behavior in women with a male twin. Van Anders et al (2005) may have an explanation: their study of finger-length ratios suggests that hormones (probably androgens) transfer between twins prenatally. In contrast, Elkadi et al (1999) found no increased incidence of left-handedness (which they claim would indicate androgen transfer) in women with male twins.

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I always wanted to be a twin, growing up. I don't have a good reason. It just seemed neat. I wasn't particularly sold on a twin of either gender, although I did think an identical twin would be kind of cool. Then again, I also thought having an identical twin would give me psychic powers.

The thing I find especially encouraging about Procopio and Marriott is that they are looking at a way in which men are more like women, rather than the ways women are more like men. It gets really exhausting reading study after study where men are the baseline condition. The male baseline is evident in all the other studies cited today.