Sex is defined as the physical differences between males and females. Primary sex characteristics are those physical characteristics that are present at birth and are directly involved
in human reproduction. In the female, the primary sex characteristics include the vagina, uterus, and ovaries. In the male the primary sex characteristics include the penis, the testes
(also called the testicles), the scrotum, and the prostate gland. Secondary sex characteristics develop during puberty and are indirectly involved in human reproduction. Female secondary
sex characteristics include a growth spurt after the first menstrual cycle, enlarged breasts, maturation of mammary glands, wider hips, pubic hair, and fat deposits on the buttocks and thighs. Male secondary sex characteristics include a deepening voice, facial chest and pubic hair, development of coarser skin texture, and a growth spurt.

Gender is defined as the psychological aspects of being feminine or masculine. Gender roles are a culture’s expectation for behavior of a person who is perceived as male or female,
and gender typing is the process by which individuals learn their expected gender role. A person’s sense of being female or male is called their gender identity and is influenced by both
biology and environment. Social learning theory proposes that individuals learn their gender identities by observing the behaviors of the people around them and being rewarded for
imitating the appropriate gender behavior. Gender schema theory suggests that children acquire their gender role by organizing their own behavior around their internalized schema
of “boy” or “girl.” A gender stereotype is a generalization about males or females that ignores individual differences.

Three landmark studies have provided much of the information available today in the U.S. about human sexuality. In 1957, William Masters and Virginia Johnson conducted the first
direct observational study on the physical aspects of the human sexual response. In 1948, Alfred Kinsey published his findings from a large survey of adult sexual behavior in the
United States. In 1993 the next large-scale study of human sexual behavior was published by Samuel Janus and Cynthia Janus.

Sexual orientation refers to a person’s sexual attraction for members of a particular sex. A sexual dysfunction is a problem with sexual functioning or the actual physical workings
of the sex act and can be caused by a number of factors. Sexually transmitted diseases or STDs are any disease that is spread through sexual contact, such as chlamydia, syphilis,
gonorrhea, genital herpes, genital warts, and AIDS (or acquired immune deficiency syndrome). AIDS is cause by the human immunodeficiency virus (HIV) which wears down the
body’s immune system, making the individual highly susceptible to infections. The virus can be transmitted from person to person through exposure to blood, vaginal fluid, semen,
and breast milk. There are no documented cases of the spread of AIDS through tears or saliva. Researchers are working on a vaccine that would prevent HIV infection.