...and twins!

Update: John Hawks has more.

Male twins reduce fitness of female co-twins in humans:

Here we 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.

Nature News has a thorough write up. Though this research doesn't explore the possible mechanism in humans, it is known in other mammals:

Fred vom Saal, a developmental endocrinologist at the University of Missouri-Columbia, points out that female mouse fetuses that develop between two male fetuses (and are thus exposed to higher levels of testosterone), are less adept at mating later in life.

"If you give a male mouse a choice between a female who hasn't been exposed to testosterone in the womb and a one who has, he will always choose the female that was exposed to the lower levels of testosterone," he says.

In addition, says vom Saal, if a male mouse does attempt to mate with them, female mice who have been exposed to prenatal testosterone are simply not as receptive to sexual advances. "They'd rather fight than have sex," he says.

So vom Saal isn't surprised that similar results have been found in humans. "If you look at the way the hormonal systems work, they are very, very similar to each other," he says.

Strangely:

Vom Saal points out that the lower levels of marriage and reproductive success does not necessarily mean that these women are unfit in an evolutionary sense. In mice, females who have been exposed to higher levels of prenatal testosterone may produce fewer offspring but, he adds, they tend to be more vigilant and protective mothers.

"The evolutionary argument is that you may have fewer kids, but there's a greater chance that they'll survive," he says.

Well, seems like this doesn't apply to humans even if it does to mice (who are born in multiples normally). Fraternal twinning ranges from 6 per thousand births in Japan to 14 and more per thousand in some African nations. Of those half will be opposite sex pairs. That's a really low frequency to imagine that some sort of evolutionary strategy could emerge under selective pressure, it seems more plausible to assume this is "developmental noise" due to the rare number of dizygotic fertilizations. Additionally, the data in the study itself suggests that these women didn't have as many grandchildren, so tautologically they just weren't that fit in an evolutionary sense.