By Lead writer: Louise Sloan
Updated: May 17, 2008

Scientists are just starting to understand the way that sex drive can be influenced by certain neurotransmitters—chemicals in the brain—mostly by observing our reactions to drugs that are intended to treat other conditions.

The most obvious example is what happens to many patients who take selective serotonin-reuptake inhibitors (SSRIs), a class of antidepressants: Their libidos shut down. SSRIs increase brain levels of the neurotransmitter serotonin, which helps regulate mood, but the drugs have also become notorious for dampening sex drive.

More about the brain-sex connection

Then there are other drugs—such as Mirapex (prescribed for restless legs syndrome) and Requip (for Parkinson's disease)—that affect levels of the neurotransmitter dopamine and often have the side effect of creating an excessive sex drive.

New theory: Sex drive is a reflex
These drug reactions serve to show us that our sex drive is at least partly a function of our body chemistry. But emotions have a major role as well. We know that, instinctively—but now scientists are starting to try to explain it.

One theory is that sex is a reflex—automatic except that your emotions can override it.

The classic reflex test is the one where the doctor hits the tendon in your knee with a mallet, and the tendon contracts, all by itself. "Let's say the doctor is trying to do this test, and outside there's a robbery with gunshots," posits Irwin Goldstein, MD, director of San Diego Sexual Medicine and the editor in chief of The Journal of Sexual Medicine. Your brain will override the reflex; your leg will stay still.

Now, Dr. Goldstein says, consider the sex reflex. "You're rubbing up against another person on a dance floor," he suggests. You'll feel desire, all right. But only "if ..." says Dr. Goldstein. "If you're not worried about safety, if it has been a nice day, if, if, if ..."

And if not? Your brain can inhibit the response and you just won't be "interested."