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"I was besotted with her." Those are the words Oscar Pistorius used in court to describe the infatuation he felt toward his girlfriend, Reeva Steenkamp, whom he shot and killed last year. Whether or not you believe the Blade Runner's story about mistaking his darling for a burglar, he has admitted to feeling jealous and possessive of her.

Of course, most men manage to keep their jealousy in check. But plenty don't. In fact, nearly all men experience the kind of infatuation Pistorius has admitted to under oath. "Crimes of passion are usually perpetrated by men," says Helen Fisher, Ph.D., a biological anthropologist and author of Why We Love: The Nature and Chemistry of Romantic Love. Men are also two-and-a-half times more likely than women to commit suicide, Fisher says, adding that, emotionally, men are often the more fragile and the more volatile of the two sexes when it comes to relationships (at least in the early stages).

While there isn't a lot of hard science on the neurology of jealousy, here's how it may mess with a man's brain if it builds and builds.

Day 1: First Week of Relationship

Studies show sex (or just the possibility of sex) triggers the release of testosterone, also known as the lust hormone. Testosterone floods the hypothalamus region of your man's brain and drives his desire to reproduce. Unfortunately, T also cranks up his aggression and possessiveness in order to scare away other suitors, Fisher says. So that explains why he may pick fights with your male friends and stare down any guy within 20 feet of you. Another cause of this early aggression may have to do with surging levels of the hormone vasopressin, which some animal studies have linked to a heightened sense of territoriality among courting males, Fisher explains.

Day 27: Fourth Week of Relationship

Your man's T levels are still elevated. And now that you're forming a closer romantic bond, Fisher says he may be experiencing euphoric brain chemicals like dopamine (which sends his energy levels and focus through the roof) and norepinephrine (which provides an emotional high). Combined with jealousy, these hormones could lead to obsessive behavior, Fisher hypothesizes. High levels of norepinephrine may also reduce his appetite if he's feeling jealous. Basically, he's "a soup" of all these different brain chemicals, which could make him an unpredictable shadow of his usual self, Fisher says.

Day 85: Third Month of Relationship, and Beyond

Although there's little research on the effects of long-term jealousy on the brain, Fisher says she wouldn't be surprised if prolonged bouts had a stress-like effect on your man's body and mind. Testosterone is a caustic substance, she says, and it could eventually stoke the release of anxiety hormones like cortisol, which has been linked to weight gain, depression, and other unhealthy drawbacks. Testosterone and cortisol may also be suppressing the release of the sleep-regulation hormone serotonin, research from the University of Pisa in Italy shows. As a result, your man not be getting solid sleep at night, which can contribute to emotional chaos. Persistently high levels of these hormones may crank up his immune system, elevating his inflammation levels, Fisher says. That could make him more likely to get sick, studies suggest.

On top of all that, some recent research from Israel has linked oxytocin to negative emotions like hate. Oxytocin is often called "the love hormone" because it spikes during new bonding phases between lovers. But it may throttle up emotional responses of all types-positive or negative-which may help explain an increasingly bitter attitude toward you, the study authors say.