Researchers, led by Indian-origin Nirao Shah of California University, have found that men with higher levels of the female sex hormone are more macho, the latest edition of the 'Cell' journal reported.
The study has found it is early oestrogen exposure that "masculinises" boys to be boys -- a phenomenon that owes more to "girl power" than was ever suspected.
Oestrogens regulate the menstrual cycle in women and cause the changes in female body shape at puberty. Testosterone causes hair growth and the deepening of voice in boys. Men and women naturally produce small quantities of both hormones, and the researchers have claimed that they are actually "sides of the same coin" in regulating many body functions.
"This really changes the way we view male and female behaviours. It's been known for decades oestrogen may play a role in making males behave like males. What we do here is to provide insight into the logic of how oestrogen regulates that behaviour," 'The Daily Telegraph' quoted Shah as saying.
The researchers have based their findings on tests on male mice -- they found that female sex hormones boosts aggression in males and those with the most are likely to pick fights and mark their territories with urine.
Dr Shah and colleagues showed brain cells with an enzyme called aromatise convert testosterone into oestrogen and men have more of them as they are critical to developing brain circuits that control male territorial behaviour.
They found more aromatise-positive cells in the males in two regions of the brain known to regulate sexual and aggressive behaviours. When female mice were given oestrogen supplements as newborns they developed brain patterns of aromatise that were indistinguishable from males.
These females now exhibited male territorial behaviour and showed aggression toward male intruders by comparison, untreated female mice rarely, if ever, attack males.
"Clearly, oestrogen was causing this male-pattern increase of aromatise-expressing cells. This suggests aromatise plays a critical role in the neural pathways responsible for these gender differences.
"About 80 per cent of male mice will fight with other males routinely, but there is always a fraction that fight poorly or not much at all," Shah said.