Why is it that a fresh, big eyed, smiling face is tonic for humans - especially for mothers?

Strathearn and his colleagues asked 28 first-time mothers with infants aged 5 to 10 months to watch photos of their own babies and other infants while they were in a functional magnetic resonance imaging scanner. The machine measures blood flow in the brain. In the scans, areas of increased blood flow "light up," giving researchers a clue as to where brain activity takes place.

In some of the photos, babies were smiling or happy. In others they were sad, and in some they had neutral expressions.

They found that when the mothers saw their own infants' faces, key areas of the brain associated with reward lit up during the scans.

The areas stimulated by the sight of their own babies were those associated with the neurotransmitter dopamine. Specifically, the areas associated included the ventral tegmental area/substantia nigra regions, the striatum, and frontal lobe regions involved in emotion processing, cognition and motor/behavioral outputs.

"These are areas that have been activated in other experiments associated with drug addiction," said Strathearn. "It may be that seeing your own baby's smiling face is like a 'natural high' ".

The study noted variation: some mothers don't get dosed as much by baby smiles. In my experience there are mothers, fathers and even total strangers that do get dosed. Something similar seems to happen when the smile is on a fresh, big eyed, smiling face of a woman rather than a baby or child. The human response may be an evolved trait that helps keep mothers from killing and eating their children, or just abandoning the demanding things, but it seems to have some more general effects and applies to men as well as women. We will work for such smiles and feel well rewarded if we get one.