PARENTHOOD can have its gross-out moments, but then so can science. In a test to examine the possible evolutionary roots of the disgust response, mothers were asked to sniff soiled diapers.

Betty Repacholi at the University of Washington in Seattle predicted that a mother would be less revolted by the smell of her own baby's faeces than those of an unrelated baby. To test this, Repacholi's team presented 13 mothers with pairs of soiled, unlabelled nappies.

The women consistently ranked their own babies' diapers as less revolting than the control diaper in the pair. While mothers may merely have become used to the smell of their own child's faeces, say the authors, they could also be picking up on some olfactory cue of relatedness (Journal of Evolution and Human Behavior, DOI: 10.1016/j.evolhumbehav.2006.03.003).

Disgust is an emotion that probably evolved to keep us from getting close to rotten or potentially unhygienic items, such as faeces, Repacholi says. "But this reaction is not going to be very adaptive if we are disgusted by our own babies."