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Playing with a baby may help shape their hormone system for future social interactions. Babies who are touched and talked to seem to develop more receptors for oxytocin during the first 18 months of life, according to a study of 101 babies and their mothers.

Oxytocin is often referred to as the “love hormone” or “cuddle chemical” because it is thought to play a role in forming relationships, in humans and other animals.

Kathleen Krol at the University of Virginia wondered how a person’s oxytocin system develops in the months after birth. Her team began by recruiting 101 mothers and their babies. When the babies were 5 months old, the team observed how each mother interacted with her baby when the two of them were left alone for 5 minutes with toys and a book.

This interaction was then scored based on how close the mother was to the baby, how she responded to the baby’s distress, and the amount of eye contact, among other things. The session was repeated when the babies were 18 months old.

Spit test

At each session, the team obtained DNA from saliva from the mothers and babies. They specifically looked at a gene that codes for the receptor for oxytocin. The team measured epigenetic modifications to this gene, which control how the gene works. These often work by small molecules that attach to DNA. The presence of a methyl group on the gene – known as methylation – suggests that the gene is “switched off”, for example.

Between the two play sessions, the mothers’ levels of methylation at the oxytocin receptor gene remained constant. But the levels changed for babies – those that had experienced more involved play with their mothers had a decrease in methylation, while those that received less attention had an increase in methylation.

This suggests that babies given more involved play have more oxytocin receptors, says Krol. These babies also appeared to have a different temperament, and were less likely to seem frustrated or overly sensitive to intense lights, sounds and textures.

Krol says there is an early window in infancy in which a baby’s environment and social interactions can impact the development of their oxytocin system. She thinks the findings are likely to apply to the way babies interact with other caregivers too. “We have no reason to believe this is specific to mothers,” she says. “My hypothesis would be that the behaviour of the father and other important caregivers is also influencing this system.”

Touch and talk

Krol thinks the findings could help shape guidance for caregivers. “The most important factors we found were things like how close the mother was to her baby… things like touch are very important, and also how talkative she was,” she says.

But it is too soon to draw parenting advice from the findings, says Emma Meaburn at Birkbeck, University of London. Krol’s team have only looked at a single region on one gene, and it is unlikely that complex human behaviour can be boiled down to this one genetic difference.

“While I think it is an interesting study, it doesn’t in its own right have practical implications for parents or caregivers,” says Meaburn. “This type of research is still in its infancy, and it is proving very hard to find clear-cut causal relationships between a child’s early experiences and social environment, and their epigenome.”