Testosterone is a steroid hormone that makes men manly. During puberty in males, it leads to the deepening of the voice and the development of body hair. It also promotes increases muscle and bone mass. Testosterone is also linked to aggressive behavior. A new study confirms this, although their definition of “aggressive behavior” seems a bit subjective. From Scientific American:

Study participants received testosterone or a placebo and then were shown color-tinted faces of men and women. The faces had subtle differences that made each one look angry, happy, or neutral.

When scientists changed the color of the face, the study subjects were asked to look at a circle of the same color below the face. Those who got the testosterone boost diverted their gaze from the angry faces more slowly than from the happy or neutral faces—an aggressive move. Subjects who got the placebo did not show this behavior.

It used to be thought that testosterone drives social dominance by imparting an awareness of feeling superior and strong. But this study shows that testosterone might be an unconscious driver, motivating social dominance as it does in other animals, involuntarily and automatically.