Would you still love your brother if you knew that because of him you've got no chance to have a healthy sex life and/or getting married? Because this is the case of girls born in mixed sex twin couples. Same sex female twin couples seem not to present this problem.

A team led by Virpi Lummaa at University of Sheffield, UK, investigated Finnish church records from 1734 to 1888, containing data of births, deaths and marriages from a pre-industrial era, without the effect of factors like advanced health care, IVF and contraception.

From the 754 cases of mixed twins, they tracked females who survived up to adulthood, and they were found to be 15 % less likely to get married, 25% less likely to have given birth, and those who did had on average two less children than women coming from same sex twin couples had. "The effects arise in utero, because females born with a male co-twin that died at birth, had similar reproductive success as twins that grew up together," said Lummaa.

This is supposed to be a hormonal effect: fetuses of both sexes receive continuously estrogen from the mother, which can diffuse across fetal membranes, but the male fetus produces testosterone, which will be received also by the female fetus sharing the womb with him.

Previous studies had shown this can induce a masculinizing effect on the facial features of a female twin, and even a male sexualization of the brain but this is the first time such an effect has been spotted on humans.

A similar, but more severe effect, has been observed in cattle: some cows, called freemartins, grow up sterile when they are the result of double gestation shared with a little bull. "Many of the effects of male co-twins on female sisters have been seen in modern populations. However, given that family size in modern Western populations tends to be very small, the effects would be difficult to pick up.", said Lummaa.

Another research made on saiga antelopes showed a reverse effect: male twins from mixed twin couples have a lower reproductive success, due to their lower birth weight, resulting in a delayed development compared to other males.