As suggested by a new research, the biological differences between male and female brains can be determined after birth.
The finding by the researchers from the University of Wisconsin, denies the earlier belief, which said that all sex-specific characteristics develop in the womb during pregnancy.
For the study, baby female rats were given treatment, which is normally reserved by mother rats for their sons.
Usually, mother rats spend more time grooming males, which, according to previous studies, is necessary for their genitalia to develop properly.
Thus, the researchers stroked baby female rats in a similar way.
It was revealed that the number of receptors for oestrogen - the female sex hormone - in the stroked rats' brains was less than in those not stroked.
Also, the oestrogen receptors were of similar levels in both female and male rats' brains.
The researchers analysed the rats' DNA, and found that among stroked females, there were more chemical "caps" on the gene controlling the number of oestrogen receptors produced.
According to the scientists, as this "capping" is often permanent, the effects of their stroking on sexual characteristics may also be long-lasting or even irreversible.
Celia Moore, from the University of Massachusetts, Boston, said the finding should act as a trigger for further research into the effects on babies' brains of human mothers treating sons and daughters differently.
"Sex may not be just genes and hormones," The Telegraph quoted her as telling the New Scientist.
The scientists said that similar trends in humans could explain why some medical conditions affect men and women differently. For example, depression is twice as common among women as in men.