Testosterone is known to increase muscle size and sex drive, but there's another equally powerful, but less well documented effect: it increases egotism.
Think about it. The average man thinks he's much better-looking than he is, whereas the average woman thinks she is less so. How many times have you heard some guy say, "Everybody tells me how good-looking I am," or words to that effect? And how many times have you met a pretty woman who seems to focus only on her own flaws? Both men and women who are at the extremes of the looks scale usually know it; it's with the people in the middle of the scale that the difference in attitude is most apparent. A woman who is a 5 will often be a self-effacing wallflower, whereas a man who is a 5, if he has sufficient testosterone on the brain, will see himself as exuding an irresistible animal magnetism.
Men always seem to think they're smarter than they are, whereas women tend to lack intellectual self-confidence. The classic example is how men will never ask for directions, but the difference manifests itself elsewhere as well. Men seem to outnumber women roughly two to one in Jeopardy (and at the tryouts as well, from what I saw.) Men are more likely to boast about their SATs, IQs, insights, and wit. And men always seem to think they should be running things.
Men take a different sort of pride in their athleticism. When Muhammad Ali first raised his arms and yelled "I am the greatest of all time!" his confidence may have been justified, but he was also giving voice to a quintessentially male self-regard. Look at the difference between Olympic swimming champions. When men win a gold medal, they tend to pump their fists and punch the water, or let out a primal yell. When women win, they tend to gape at the scoreboard in wide-eyed and ecstatic disbelief, then hug the girl in the next lane. I've never seen a woman -- even the ones on steroids -- hammer fist the water in exultation.
When an Olympic men's team wins, they tend to chant, "USA! USA! USA!" When a collegiate men's team wins the NCAA championships, they tend to chant, "We're number one! We're number one!" When a women's team wins, they tend to jump up and down and squeal with delight.
After having sex for the first time with a new partner, a woman's instinct is to cuddle. A man's instinct is to place his foot on the woman's chest and let out a Tarzan yell, then use her bedside phone to call his buddies and tell them he's scored. (Most men know better than to act on those instincts.)
Men swagger. Men exult. Men boast. Men bluster. These are not verbs one tends to associate with women.
You never hear a female cry out, "I'm the woman!"
For most women, the ideal life would consist of an endless group hug. For most men, it would be a nonstop end zone dance.
A certain amount of egotism can be beneficial -- you need some to have the confidence to attempt various things. The meek may inherit the earth, but only after the grotesquely egotistical have run roughshod over them and proclaimed their right to it first.
Testosterone seems to spur the brain into indulging in egotistical reveries. Estrogen seems to spark the thought, I really need that pair of shoes.
Hard to figure out which is worse.