If you crave chocolate, you're not alone. Approximately 50 percent of women in North America experience chocolate cravings, half of them around the time of menstruation. Chocolate craving is learned behavior, passed from generation to generation and intensified after chocolate is eaten when hungry. But what causes it? Surprisingly, little research exists to answer this question. Chocolate contains several bioactive substances that may affect serotonin and dopamine levels in the brain, chemicals known to influence mood. Plus, there's no denying the sensuous taste of chocolate melting in your mouth and stimulating the brain's pleasure chemicals, the endorphins. While some stodgy scientists question whether chocolate actually changes brain chemistry, millions of people can attest to the cocoa bean's uncanny ability to provide solace and pleasure.