Ever wondered if you might be adopted? Try giving your brothers and sisters a good sniff. That advice arises from new research that shows that family members can tell each other by smell alone, but only if they are genetically related to each other.

Recognising close relatives by their odour could be down to familiarity, or simply sharing a common environment and common smells. But psychologist James Gall of Wayne State University in Detroit and his colleagues compared members of families in which both blood relatives and step children lived, to see if the genetic link made a difference.

The results showed that mothers were very good at detecting their birth children by smell, but not their step children. The children themselves were quite good at distinguishing their brothers and sisters over their step siblings.

“We have a basic affinity for our biological relatives,” says Gall’s colleague, Glenn Weisfeld. He says smell could explain why stepchildren are often more badly treated than children living with their biological parents. “I think it’s an important mechanism in how people discriminate against stepchildren,” he says.


Sealed bag

In the study, 34 pairs of siblings aged four to 11 took part – 13 full siblings, 10 half siblings and 11 step siblings. All of the pairs had been living in the same home with their parents for at least the last two years.

The children were given a clean T-shirt, and asked to wear it for three nights in a row. During the day, the shirt was kept in a sealed plastic bag to protect it from being contaminated with household smells. The children were all also given the same unperfumed soap to use for the three days.

At the end of the three nights, each shirt was put into a container with a small opening in the top for smelling through. Each mother was given two shirts and asked which smell she preferred, and which she thought was from her own child.

The biological mothers knew which was their child 27 times out of 30. But the stepmothers were wrong five times out of seven.

Because the mothers had all been living with their children, the results mean they must be using smell to recognise their children’s genes, Weisfeld says: “Similar environments and close association are not always sufficient.”

Telling smell

The researchers also found that the biological mothers preferred the smell of their own children over that of unrelated kids.

The results were the similar for the children. They recognised the smell of full brothers and sisters 21 times out of 30. Half siblings were correct 16 times out of 28, while step siblings performed worse than chance.

Previous studies have found that blindfolded mothers can identify their newborn babies, and that women smelling men’s T-shirts find the odour of men less related to them more attractive. But Weisf eld says this study is the first to compare full siblings with half and step siblings. “It’s the degree of kinship that’s important,” he says.

This research was presented at the Human Behaviour and Evolution Society conference in London, UK.