Having a baby alters new mothers' brain activity, according to a study, which has also found the first evidence of similar changes in the brains of gay men raising children adopted through surrogacy.
The gay men's pattern of brain activity resembles that of both new mothers and new fathers in the study.
The Israel-conducted research builds on work which showed that the brains of new mothers become hyper-reactive to their child's cries and other emotional cues.
It was not clear if that pattern is a result of hormonal changes that accompany pregnancy or a response to the experience of motherhood.
The researchers videotaped 89 new mothers and fathers interacting with their infants at home.
They then measured the parents' brain activity while watching the videos in an MRI tube, and again to establish a baseline while watching videos that their children did not star in.
In the 20 mothers in the study, watching their babies triggered heightened activity in the brain's emotion-processing regions, the scientists discovered.
Heterosexual fathers were found to be very involved in raising their baby but their wives took the parenting lead - watching their infant increased activation of cognitive circuits.
The 48 gay fathers seemed to be both mother and father brain-wise, the study found. Their emotional circuits were as active as those of mothers and the interpretive circuits showed the same extra activity as that of heterosexual fathers.
"Fathers' brains are very plastic," said neuropsychologist Ruth Feldman. "When there are two fathers, their brains must recruit both networks, the emotional and cognitive, for optimal parenting."