1. In men, testosterone waxes and wanes throughout the day, peaking in the morning for younger guys.

2. Husbands have markedly lower testosterone levels than bachelors.

3. Playing competitive sports increases it.

4. Holding a baby, even a doll, decreases it.

5. Testosterone dampens the activity of the immune system.

6. But it may boost cognition. The higher the level of free testosterone in older men, the less likely they are to suffer certain kinds of memory loss.

7. Synthetic testosterone could be the next male contraceptive, suppressing sperm development without dulling sex drive.

8. Many men are on testosterone replacement therapy to stay young, although it hasn't been proven effective or safe. Concerns include increased risk of prostate cancer and stroke. (Sound familiar?)

9. Women have testosterone, too, though only one seventh of the amount coursing through a man, and it powers our sex drives along with estrogen.

10. Two hormone-replacement pills for menopausal symptoms (Estratest and Premarin plus methyltestosterone) contain testosterone to help increase sex drive.

11. Although a few million women may be taking testosterone to increase libido, there's no long-term safety data, according to hormone expert Susan Love, MD, author of Self Nuture and one of our Lluminari experts. In fact, Love says, blood levels of testosterone are correlated with an increase in breast cancer risk.

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