( en increases aggression in short-day mi...)Changing length of days reverses how estrogen affects aggressiveness in mice

"In the vast majority of cases, hormones seem to affect behavior by working through genomic pathways, so it is always interesting when you find something different," Nelson said. "This seems to be one of those instances where estrogen is working in a different way in long-day mice. But there is a lot more work to be done on this."

These findings have many implications for humans, according to the researchers. For one, it suggests more work needs to be done to determine the role estrogen plays in aggression in humans. In general, estrogen works to inhibit aggression in humans, but this study suggests research needs to look more at the role of estrogen receptors in some parts of the brain, Nelson said.

Also, scientists are very interested in understanding how estrogen works at the molecular level in humans, especially its role in promoting cancer.

"A lot of the research looks at how genes and hormones work in a controlled environment outside the body. But this study shows that the environment can play a very significant role in how estrogen reacts in mice," Trainor said.

"If something as simple as the length of day can affect how estrogen is used in the body, at least in some species, how are other environmental factors such as diet affecting estrogen in humans? It is something we don't know enough about."