Although many scientists believe that the same system - involving the vomeronasal organ (VNO) - is non-functional in humans - others think that it may well play a role in behaviour.

The research, published in the journal Science, found a gene in mice which is vital to keep the vomeronasal system going.

When the scientists, from Harvard University in the US, bred mice missing the vital gene, their behaviour greatly changed.

The altered male mice were less likely to become aggressive when another male was placed in their cage.

In fact, they were just as likely to initiate courtship or try to mate with either males or females.

The scientists suggested that the VNO, which although it can be found in the nose, is quite distinct from other olfactory parts, and leads to a different part of the brain, must help animals such as mice spot the difference between males and females.

Controversial theories

While the role of the VNO is easier to determine in animals, in humans it is far more complex.

The vast majority of humans do have one - it can be found on the nasal septum and is only 0.5mm across.

However, scientists have so far failed to find any conclusive evidence that it is functional in adult humans.

It appears to work in foetuses, and perhaps even in newborns, but then regresses as the baby grows.