People do not like the smell of close family members, new research shows - the aversion might help prevent incest.

People can recognise the smell of their close family members - but surprisingly, they do not like it. This aversion may help prevent incest, the discoverers speculate.

Most studies of smell recognition in humans have looked at mothers and their newborn babies, who learn to recognise each other's smell soon after birth. But Tiffany Czilli's team at Wayne State University in Detroit wanted to know how well all the other members of the family would fare in a sniff test.

She recruited 25 families with at least two children aged between 6 and 15, and gave the participants odourless T-shirts, odourless soap and resealable bags with their names on them. The volunteers were instructed to sleep in the T-shirts for three nights, wash only with the soap provided and seal the shirts into the plastic bags each morning.

Czilli then asked everyone to sniff two T-shirts, one worn by a family member and one by an unknown and unrelated person. She asked fathers and mothers if they could detect the scent of their children, and children if they could identify their parents or siblings. She also asked each of the participants which odour they preferred.

Both mothers and fathers recognised their pre-adolescent offspring, Czilli found, though mothers tended to be more accurate. Neither parent could distinguish between their children, however.

Strong aversion

For their part, pre-adolescent children aged 5 to 8 did not recognise their mother's smell, while older children, aged 9 to 15, did. Breastfed sons were the exception: they could recognise their mothers. And all the children recognised dad's smell.

When asked which smells they liked, the answers were even more intriguing. The volunteers far preferred the smells of other people to those of their own family members. Mothers especially disliked their children's smells, while children had a strong aversion to their dad's scent. "Recognition and preferences can operate independently," Czilli says.

She thinks that disliking the smell of close family might be part of the mechanism that helps prevent incest. Particularly notable is the fact that opposite-sex siblings disliked each other's smells, while same-sex siblings did not. She also speculates that the aversion to dad's odour could reflect social distancing and independence in the child.

The avoidance of inbreeding could definitely be at work, says Dustin Penn at the University of Utah in Salt Lake City. But he warns that asking people about their preferences can be unreliable.

"Just because people say they 'prefer' something doesn't mean they'll act in a preferential way," he says. Smell preference is context dependent, he adds. Liking someone's smell doesn't always mean you'd like to sleep with them.