The Volokh Conspiracy - More on "Biology Is Destiny,"

though fortunately that's usually shorthand for "destiny has a lot more to do with biology than we might like": From the abstract of an article linked to by Slate's fascinating Human Nature column (some paragraph breaks added):

In mammals, including humans, female fetuses that are exposed to testosterone from adjacent male fetuses in utero can have masculinized anatomy and behavior.... [W]e investigate the effects of being gestated with a male co-twin for daughter lifetime reproductive success, and the fitness consequences for mothers of producing mixed-sex twins in preindustrial (1734-1888) Finns.

We show that daughters born with a male co-twin have reduced lifetime reproductive success compared to those born with a female co-twin. This reduction arises because such daughters have decreased probabilities of marrying as well as reduced fecundity. Mothers who produce opposite-sex twins consequently have fewer grandchildren (and hence lower fitness) than mothers who produce same-sex twins.

Our results are unlikely to be a consequence of females born with male co-twins receiving less nutrition because such females do not have reduced survival and increases in food availability fail to improve their reproductive success. Nor are our results explained by after-birth social factors (females growing up with similarly aged brothers) because females born with a male co-twin have reduced success even when their co-twin dies shortly after birth and are raised as singletons after birth.

Our findings suggest that hormonal interactions between opposite-sex fetuses known to influence female morphology and behavior can also have negative effects on daughter fecundity and, hence, maternal fitness, and bear significant implications for adaptive sex allocation in mammals.

The Slate piece goes on to summarize the numbers, from the body of the paper (paragraph breaks added):

Gestating with a male twin makes a girl 25 percent less likely to become a mother than if her twin is female .... Women whose twins were male were 15 percent less likely to marry; those who became parents had two fewer children....

Explanatory speculations: 1) "Perhaps the female twins had more masculine attitudes and behaviors that affected their decision to get married." 2) "Male features could have made the women less attractive to mates." 3) "Exposure to elevated levels of testosterone during development can promote diseases that compromise fertility, such as reproductive cancers."

Bonus finding: Gestating with a female twin had no such effect on boys.

Caveat: The study was based on church records from two centuries ago, "to obtain results that are not affected by advanced health care and contraception." (For a previous update on differences between lesbian and heterosexual brains, click here.)

	[Eugene Volokh, June 19, 2007 at 5:26pm] Trackbacks More on "Biology Is Destiny," though fortunately that's usually shorthand for "destiny has a lot more to do with biology than we might like": From the abstract of an article linked to by Slate's fascinating Human Nature column (some paragraph breaks added): In mammals, including humans, female fetuses that are exposed to testosterone from adjacent male fetuses in utero can have masculinized anatomy and behavior.... [W]e investigate the effects of being gestated with a male co-twin for daughter lifetime reproductive success, and the fitness consequences for mothers of producing mixed-sex twins in preindustrial (1734-1888) Finns. We show that daughters born with a male co-twin have reduced lifetime reproductive success compared to those born with a female co-twin. This reduction arises because such daughters have decreased probabilities of marrying as well as reduced fecundity. Mothers who produce opposite-sex twins consequently have fewer grandchildren (and hence lower fitness) than mothers who produce same-sex twins. Our results are unlikely to be a consequence of females born with male co-twins receiving less nutrition because such females do not have reduced survival and increases in food availability fail to improve their reproductive success. Nor are our results explained by after-birth social factors (females growing up with similarly aged brothers) because females born with a male co-twin have reduced success even when their co-twin dies shortly after birth and are raised as singletons after birth. Our findings suggest that hormonal interactions between opposite-sex fetuses known to influence female morphology and behavior can also have negative effects on daughter fecundity and, hence, maternal fitness, and bear significant implications for adaptive sex allocation in mammals. The Slate piece goes on to summarize the numbers, from the body of the paper (paragraph breaks added): Gestating with a male twin makes a girl 25 percent less likely to become a mother than if her twin is female .... Women whose twins were male were 15 percent less likely to marry; those who became parents had two fewer children.... Explanatory speculations: 1) "Perhaps the female twins had more masculine attitudes and behaviors that affected their decision to get married." 2) "Male features could have made the women less attractive to mates." 3) "Exposure to elevated levels of testosterone during development can promote diseases that compromise fertility, such as reproductive cancers." Bonus finding: Gestating with a female twin had no such effect on boys. Caveat: The study was based on church records from two centuries ago, "to obtain results that are not affected by advanced health care and contraception." (For a previous update on differences between lesbian and heterosexual brains, click here.)