Does testosterone cause aggression? What does testosterone do to us poor males anyway? After all, if you look at all that goes in planet earth, it's pretty obvious that us males are responsible for lion's share of violent crime, war and other nasty stuff. Is testosterone somehow to blame?
In animals the answer is a pretty clear 'yes'. High levels of testosterone make furry and feathered critters do strange things. For example, if you give testosterone to lizards, they "get stupid" and start to make themselves conspicuous and more vulnerable to predators.  You can give birds extra testosterone and find that they will ascend higher in the pecking order.  Many other studies have tied testosterone to aggression in animals as well.
But what humans and, specifically, human males? Here the conclusions become much less, well, conclusive. It's fairly obvious that testosterone plays some role in aggressiveness and assertiveness in males - just watch what happens to castrated males. Some studies show that eleveated testosterone levels from steroids can lead to very aggressive behavior. In fact, one study showed that some normal, healthy subjects developed manic behavior when give steroids.  It was only a few individuals, but it does show that some men will "go crazy" if you give them enough testosterone.
However, castrated males and steroid users are decidely the two extremes. What about men in normal physiological ranges? Does a high normal man generally have more aggressiveness than a normal man? This has actually been studied quite extensively and some studies said yes and still others no.
Fortunatly, there were sufficient trials to where researchers could do a meta-analysis on all the studies in toto. The latest large study in 2001 and a previous one both found came to the same conclusion: there is a "weak, positive relationship" between testosterone levels and aggression.  In other words, on average testosterone makes males slightly more aggressive.
So, bottom line, is that we cannot really blame testosterone for bad behavior. Interestingly enough, researchers have found that estrogen/artomatase likely plays just as much of a role along with certain stress hormones.
What about men with very low (hypogonadal) testosterone? Are they particularly sensitive to the extra testosterone in a negative way? The studies show no significant increase in aggressive behavior in this group either.  In fact, this same study showed a decrease in many negative emotions, i.e. "significant reductions in negative mood (tension, anger, and fatigue) followed by an increase in vigor were found in response to T treatment in the hypogonadal group".  Still other studies have shown that Hormone Replacement Therapy improves mood and outlooks as well.
1) BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY, 23(1):21-26, "Evolutionary costs of aggression revealed by testosterone manipulations in free-living male lizards"
2) Rev. Bras. Cienc. Avic. Jan./Mar. 2006, 8(1), "Aggressive behavior in the genus Gallus sp"
3) Arch Gen Psychiatry. 2000, 57:133-140, "Effects of Supraphysiologic Doses of Testosterone on Mood and Aggression in Normal Men"
4) Aggression and Violent Behavior, Nov-Dec 2001, 6(6):579-599, "The relationship between testosterone and aggression: a meta-analysis"
5) Physiol Behav, 2002 Apr 1, 75(4):557-66, "Exogenous testosterone, aggression, and mood in eugonadal and hypogonadal men"