Gender refers to the socially influenced characteristics by which people define men and women. Sex refers to the biologically influenced characteristics by which people define male and female. Our understanding of gender arises from the interplay between our biology and our experiences.

Males and females are more alike than different, thanks to our similar genetic makeup—we see, hear, learn, and remember similarly.

Male-female differences include body fat, muscle, height, age of onset of puberty, life expectancy, and onset of certain disorders.

Men admit to more aggression than women do, and they are more likely to be physically aggressive. Women are slightly more likely than men to commit relational aggression.

In most societies, men have more social power, and their leadership style tends to be directive, whereas women’s is more democratic. Women often focus more on social connectedness than do men, and they “tend and befriend.”

Gender bias in the workplace is seen in such differences as perception, compensation, and child-care responsibility.

Social norms, leadership styles, interaction styles, and everyday behaviors also contribute.

Both sex chromosomes and sex hormones influence development.

The twenty-third pair of chromosomes determines sex, with the mother contributing an X chromosome and the father contributing either an X chromosome (for a girl baby) or a Y chromosome (for a boy baby). A Y chromosome triggers additional testosterone release and the formation of male sex organs.

During puberty, both primary and secondary sex characteristics develop. Sex-related genes and physiology influence behavioral and cognitive gender differences between males and females.

Intersex individuals are born possessing biological sexual characteristics of both sexes.

Gender roles describe how others expect us to act and vary depending on cultural expectations, which change over time and place.

Social learning theory proposes that we learn our gender identity—our sense of being male, female, or some combination of the two—as we learn other things: through reinforcement, punishment, and observation. But critics argue that cognition also plays a role because modeling and rewards cannot explain variation in gender typing.

Some children organize themselves into “boy worlds” and “girl worlds”; others prefer androgyny.

Transgender people’s gender identity or expression differs from that associated with their birth sex. Their sexual orientation may be heterosexual, homosexual, bisexual, or asexual.

The female estrogen and male testosterone hormones influence human sexual behavior less directly than they influence sexual behavior in other species.

These hormones direct sexual development in the prenatal period; trigger development of sexual characteristics in adolescence; and help activate sexual behavior from puberty to late adulthood.

Women’s sexuality is more responsive to testosterone level than to estrogen level. Short-term shifts in testosterone level are normal in men, partly in response to stimulation.

William Masters and Virginia Johnson described four stages in the human sexual response cycle: excitement, plateau, orgasm (which involves similar feelings and brain activity in males and females), and resolution. Males then enter a refractory period in which renewed arousal and orgasm are impossible.

Sexual dysfunctions are problems that consistently impair sexual arousal or functioning. They can often be successfully treated by behaviorally oriented therapy or drug therapy.

Safe-sex practices help prevent sexually transmitted infections (STIs). Condoms are especially effective in preventing transmission of HIV, the virus that causes AIDS.

A vaccination administered before sexual contact can prevent most human papilloma virus (HPV) infections.

Erotic material and other external stimuli can trigger sexual arousal in both men and women.

Viewing sexually coercive material can lead to increased acceptance of violence toward women. Viewing sexually explicit materials can cause people to perceive their partners as comparatively less appealing and to devalue their relationships.

Imagined stimuli (fantasies) help trigger sexual arousal.

Rates of teen intercourse vary from culture to culture and era to era.

Factors contributing to teen pregnancy include minimal communication about birth control with parents, partners, and peers; impulsive sexual behavior, with passion overwhelming self-control; alcohol use; and mass media influences and social scripts.

High intelligence, religious engagement, father presence, and participation in service learning programs have been predictors of teen sexual restraint.

Sexual orientation is an enduring sexual attraction toward members of one’s own sex (homosexual orientation), the other sex (heterosexual orientation), or both sexes (bisexual orientation).

About 3 or 4 percent of men and 2 percent of women are homosexual. Sexual orientation is enduring.

Sexual orientation is not an indicator of mental health. There is no evidence that environmental factors influence sexual orientation.

Evidence for biological influences on homosexuality comes from same-sex attraction in many animal species; gay-straight trait and brain differences; genetic influences; and prenatal influences.

Evolutionary psychologists attempt to understand how natural selection (how nature selects traits and appetites that contribute to survival and reproduction) has shaped behaviors found in all people.

They reason that men’s attraction to multiple healthy, fertile-appearing partners increases their chances of spreading their genes widely. In contrast, women tend to be choosier than men because of their need to incubate and nurse babies. Women increase their own and their children’s chances of survival by searching for mates with the potential for long-term investment in their joint offspring.

Critics argue that evolutionary psychologists (1) start with an effect and work backward to an explanation, (2) do not recognize social and cultural influences, and (3) relieve people from taking responsibility for their sexual behavior.

Evolutionary psychologists respond that understanding our predispositions can help us overcome them. They recognize the importance of social and cultural influences, but they also cite the value of testable predictions based on evolutionary principles.

Scientific research on human sexuality does not aim to define the personal meaning of sex in our own lives, which is influenced by many social factors. Sex is a socially significant act. Intimacy expresses our social nature, and sex at its human best is life uniting and love renewing.

Our ancestral history helped form us as a species. Our genes form us, but our culture and experiences also shape us. Nature and nurture interact in the development of our gender-related traits and our mating behaviors. We are both the creatures and creators of our worlds, with our own hopes, goals, and expectations directing our future.