NARRATOR: Birds and bees do it with seeming abandon. But for humans, sex is a more complicated act that balances instinct and intimacy.

HEATHER BERLIN: We have evolved the desire to have sex because it's how we procreate. It's an evolved function so that we can survive.

NARRATOR: Adults find each other sexually attractive for all sorts of reasons. Genetic, cultural, and biological factors can all play a part, as well as more subjective qualities such as physical attractiveness, personal style, or vocal quality. And some aspects of attractiveness overlap.

HEATHER BERLIN: There are a lot of things that attract you to a person that are occurring outside of awareness. For example, smell.

ZHANA VRANGALOVA: People seem to be attracted to the smell of those whose immune systems are somewhat different from their own because that baby will have immune protection against a greater variety of diseases.

NARRATOR: Hormones, chemical messengers that travel throughout the bloodstream to regulate our physiology and behavior, are the basic building blocks of human sexual behavior. Testosterone is the most important male sex hormone, and estrogen is the most important female sex hormone. During sexual arousal hormone levels rise.

ZHANA VRANGALOVA: Hormones effect both attraction and arousal in a number of different ways. Our heart starts beating faster, blood rushes to the genitals, our hair rises, our nipples get harder, there is lubrication in the walls, there is a reaction in men. That all prepares the body for sexual activity.

NARRATOR: In the 1960s pioneering sex researchers William Masters and Virginia Johnson proposed a four stage cycle of human sexual response. During the first stage, excitement, the body prepares for sex, heart rate and breathing increase, and blood rushes to the genitals. During the second stage, plateau, tension builds that is released during the third stage, orgasm, the climax of sexual activity. The final stage is resolution, as the body comes down from its excited state.

Masters and Johnson were also interested in identifying and treating sexual dysfunction, problems that interrupt sexual desire, arousal, or response, such as difficulty maintaining erection for men or achieving orgasm for women. A range of treatment is available to tackle sexual dysfunction, including sex therapy, a form of behavior therapy that focuses on intimacy and sexual problems.

ZHANA VRANGALOVA: Sexual disfunctions are actually pretty common. There are all of these various issues that might come up, for example, low sexual desire, pain during intercourse, or premature ejaculation. Some of these issues may be resolved with medicine or maybe surgery or that kind of stuff, but a lot of it is psychotherapy.

NARRATOR: Cultural influences and societal expectations also play a large part in what we find attractive and how we express our sexuality. Sex exemplifies the fundamental connection between our minds our bodies and the world around us.

ZHANA VRANGALOVA: There's no one size fits all. As long as you're being honest about what you want, you're communicating that well with your partner, and you are getting needs met without harming your partner in the process, that makes for a healthy relationship.