Men who were exposed to more testosterone in the womb before birth may behave more cordially to women as adults, according to a study published in the journal Personality and Individual Differences.

Research suggests that the amount of testosterone present in the fetal environment before birth affects many aspects of behavior later in life for both men and women, including sexual orientation and aggression.

A good indicator of the approximate level of fetal testosterone exposure can be found by comparing the length of the index and ring fingers (known as the 2D:4D ratio). A low 2D:4D ratio (index finger shorter than ring finger) indicates high exposure to testosterone in utero, while a high 2D:4D ratio (ring finger longer than index finger) indicates low exposure.

A team of scientists led by D. S. Moskowitz, of McGill University, used measurements of the 2D:4D ratio to conduct a study of how fetal testosterone exposure affects male-female interactions in adulthood. The sample for the study included 155 people who were recruited in male-female and male-male pairs (examples include coworkers, siblings, and romantic partners). Participants reported on their interactions with the other members of these pairs each day for 20 days, recording how often they had engaged in agreeable interactions (such as smiling) and quarrelsome ones (such as being sarcastic). The participants’ 2D:4D ratios were also measured.

Men were more agreeable in their interactions with women than in their interactions with men. However, the size of this gender gap in agreeable behavior was larger among men with low 2D:4D ratios than among those with high 2D:4D ratios. Both groups of men were equally agreeable in their interactions with men, but those with low 2D:4D ratios were significantly more agreeable in their interactions with women.

Meanwhile, men with high 2D:4D ratios were equally quarrelsome with men and women, but those with low 2D:4D ratios quarreled less with women than they did with men. Women’s interactions did not differ as a function of 2D:4D ratio, or on the basis of the gender of their interaction partners.

“Our results suggest that greater exposure of the fetal male brain to androgens may produce changes that enhance how agreeable adult men are to women, but not to men,” Moskowitz and his colleague wrote.

The authors conclude that men who experienced higher levels of testosterone exposure before birth (as indicated by low 2D:4D ratio) tend to favor women to a greater extent in their adult social interactions than men exposed to lower testosterone levels. They suggest that prenatal testosterone exposure affects fetal brain development in ways that affect motivation to make social connections with women in adulthood.