Scientists find that moms consistently rank the stink of their baby's "number two" as No. 1.

In a new study, 13 mothers were asked to sniff soiled diapers belonging to both their own child and others from an unrelated baby. The women consistently ranked the smell of their own child's feces as less revolting than that of other babies.

This effect persisted even when the diapers were purposely mislabeled.

One possible explanation is that the mothers were simply more accustomed to their their baby's stink and therefore found it less repulsive. A more intriguing possibility, the researchers say, is that the mothers' reactions are an evolutionary adaptation allowing them to overcome their natural disgust so that they can properly care for their babies.

The study, led by Trevor Case of Macquarie University in Australia, will be detailed in an upcoming issue of the journal Evolution and Human Behavior.

Disgust helps protect us from things that can make us sick, scientists say, but there are circumstances in which the emotion has to be tempered. The caring of an infant is likely one such instance.

"A mother's disgust at her baby's feces has the potential to obstruct her ability to care for her baby and may even affect the strength of the bond she has with her baby," the researchers write.

The finding is among the latest in a series of studies suggesting that humans can determine biological relatedness through body odor. Another recent study found that mothers more accurately identify and prefer the smell of their biological children over that of stepchildren.