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The hormone testosterone doesn't always have the best reputation. Learn why and find out about the difference between men's and women's brains. See more football pictures.

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As hormones go, testosterone doesn't always have the best reputation. It's a key ingredient in most anabolic steroids, which can lead to aggression and infertility. A 2006 study showed that too much of it can kill brain cells [Source: CNN]. And some women hold it accountable for just about any stereotypically masculine behavior, from a love of sports to a reluctance to ask for directions.

But without testosterone, no one would ever grow into a male body. Until about the eighth week of gestation, males and females are identical. Then, a surge of testosterone causes male fetuses to develop male sex organs -- without this wave of testosterone, a male's sex organs will never develop.

As he grows, testosterone continues to play an important part in his physical development. When he's a teenager, the hormone's presence causes him to develop masculine secondary sex characteristics, such as facial and body hair and a deeper voice. Testosterone also encourages his adult body to be taller and heavier than a woman's, with broader shoulders and less subcutaneous fat. In other words, testosterone plays a big part in making a male body physically different from a female one.

But the influence of testosterone doesn't stop with outward appearance. It also affects the brain. Researchers theorize that the same wave of testosterone that affects males in the womb also causes men's brains to be different from women's brains. Men and women are equally intelligent, and their brains are the same size compared to the rest of their bodies. But male and female brains have different proportions of gray and white matter. Men have more gray matter and less white matter than women do [Source: LiveScience].

Gray matter contains the brain's processing centers, and white matter creates networks between areas of gray matter. In other words, men's brains have more areas in which to process data, and women's brains have more networks between processing centers. Some researchers believe this supports the common stereotype that men are better at science and math while women are better at language. It appears that men's brains are set up to process concrete data, while women's brains are wired for making connections.

But, according to many studies, this stereotype isn't true at all. Although men and boys usually score better on standardized math tests, women and girls usually make better grades in math [Source: Psychology Today]. After performing a meta-analysis of data from lots of studies, some researchers now believe that the differences in men and women's abilities in math and language are so slight that they're insignificant [Source: Economist].

The idea that men are better at math and science than women are is one of many stereotypes that doesn't hold up well under scientific scrutiny. In this article, we'll explore whether other common perceptions about men are accurate.