Breastfeeding might affect the way babies with a certain genetic makeup perceive other people's emotions, according to a new study.

In the study, researchers looked at the relationship between being breastfed and perceiving emotions in 49 female and 49 male 7-month-old infants. The babies were shown photos of faces whose eyes expressed emotions including happiness or anger, and the researchers measured how long the babies looked at them.

The researchers found that among the 44 babies in the study who had a certain genotype of the gene CD38, called the CC genotype, those who had been exclusively breastfed for the longest time tended to look longer at happy eyes, and for less time at angry eyes, compared with infants who had been exclusively breastfed for a shorter time.

The results show that these breastfed infants seemed to be more sensitive to the social cues expressed in people's eyes, the researchers said.

One important cultural aspect of the study was that it was conducted in Germany, and not in the United States. Most mothers there are entitled to a one-year-long paid maternity leave, making it easier for moms to breastfeed if they choose to, said study author Tobias Grossmann of the University of Virginia in Charlottesville. Almost all of the mothers in the study were still on maternity leave when the study was conducted, he said.

The goal of the new research is not to put pressure on mothers to breastfeed their children, but rather to examine potential biological mechanisms connected to breastfeeding and babies' social behaviors, Grossman stressed. "It is very important not to put any stigma on what mothers do," he noted. [7 Baby Myths Debunked]

For the babies in the study who didn't have this CC genotype, the length of time they were breastfed was not linked with how long they looked at the emotional faces, according to the study.

The link seen in the study appears to involve oxytocin, a hormone associated with social bonding. Previous research on humans has shown that the changes in the CD38 gene may be associated with lower levels of oxytocin in the brain, and that people with these changes may have impaired social skills, the researchers said.

The new results suggest that, through acting as an external source of oxytocin, breastfeeding may help to regulate the oxytocin levels in the infants with this genotype and improve their social skills, the researchers said.

The specific genotype in the babies in this study has also been linked to an increased risk of autism, the researchers said. The new findings suggest that the "breastfeeding experience enhances prosocial tendencies in infants that are genetically at risk for autism," they wrote in the study.

It is not clear whether this effect of breastfeeding may disappear once the babies are weaned, or whether it persists and eventually affects the way the children behave later in life, Grossmann said. The researchers are currently carrying out a larger project to investigate this question, he said.

The new study was published today (Sept. 14) in the journal Proceedings of the National Academy of Sciences.

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