The Biology of the Orgasm

Editors note: In this article, we note the differences between the “male” and “female” orgasm and use biological sex as the distinction between the two. Due to the heteronormative nature of the research conducted on this topic, the information in this article does not reference intersex individuals. However, orgasm is an experience that can be achieved by many individuals regardless of genitalia and we encourage readers to use this article to become interested in what may be happening to their body during orgasm.

Sexual arousal and orgasm are extremely complex biological functions, affecting numerous areas of the body from the skin to the brain. To make matters even more complicated, so much about the orgasm differs between the sexes—from the how, to the why, to the when. In addition to the physiological intricacies, sexual desire, arousal and orgasm are often social activities that reflect the complexities of intimate human interaction. Although the orgasm may never be fully understood from a psychological, physiological or evolutionary standpoint, researchers have come a long way in helping us better understand this fascinating and popular event.


There are thought to be four stages of sexual arousal: excitement, plateau, orgasmand resolution.


This stage may be initiated by numerous types of stimuli, including visual, physical and psychological stimulations. These stimulations lead to an increase of parasympathetic—the relaxed portion of the autonomic, involuntary nervous system—activity along the pelvic nerves. For many individuals, the heart rate quickens, skin may become flushed and muscle tension increases. 

For males, it is during this stage that the spongy tissue becomes erect. A penile erection is caused by the release of nitric oxide from neurons into the arterial blood supply of the penis. This chemical compound causes the blood vessels to relax and allows the tissue to become engorged with blood. Also, during this stage, the tip of the penis becomes lubricated with secretions of the blubo-urethral glands and the testicles tighten.

In females, the clitoris becomes erect and the vagina becomes lubricated. There is debate over how the clitoris becomes engorged with blood, but the presence of nitric oxide indicates that it may utilize a mechanism similar to a penile erection.

Unlike the penis, the vagina may not become lubricated due to the function of a specific gland. At the time of publication, there is some debate about the role that the secretions of the Bartholin glands play in vaginal lubrication. The Bartholin glands are two pea-sized glands located on the left and right sides of the vaginal opening. Historically, these glands were thought to produce compounds that lubricated the vagina; however, current research shows that the secretions of the Bartholin glands may be relatively minute.  Many researchers believe that the majority of vaginal lubrication is a combination of blood vessel transudate—a fluid substance that has leaked out of the blood supply—and cervical mucus.  Also, during this stage, the breasts may become fuller and the nipples erect.

All of this occurs in mere seconds, and without any conscious thought. So far, human arousal has proven to be a complex series of events, perfectly coordinated. As we’ll see, things are about to become even more interesting.


During this stage, many of the functions of the excitement stage are maintained, such as erection and lubrication. The clitoris may become highly sensitive and retract under the clitoral hood to avoid direct simulation; in addition, the vaginal walls turn dark purple. In males, the testicles are drawn up into the scrotum. Some individuals may experience muscle spasms in the hands, feet or face as muscle tension increases. Heart rate continues to quicken, blood pressure rises and the individual becomes closer to orgasm.


After excitation and the plateau stages comes the phase of sexual arousal that, for many individuals, serves as the primary motivation for sexual interaction. Typically lasting just a few seconds, this phase is by far the shortest. In males, contractions of the prostate and seminal glands produce secretions that enter the urethra and mix with sperm to create semen. Ejaculation occurs as the muscles of the penis—specifically the ischiocavernosus and bulbocavernosus—begin to rhythmically contract. In females, the muscles of the vagina and uterus also produce powerful, rhythmic contractions. These contractions are the source of pleasure during orgasm, as they release muscle tension built up during the excitement and plateau phases.


After orgasm, heart rate, blood pressure and breathing return to normal. Many individuals experience a relaxed feeling during this period and fatigue is common. During the resolution stage, most males will not be able to achieve sexual arousal and orgasm, although some females are able to. 


As we’ve seen, from a physiological stand point sexual arousal is an intricate, complicated process that affects many areas of the body. As complex as the “how” is, the “why” is even more convoluted. From an evolutionary standpoint, understanding the male orgasm is relatively straightforward; ejaculation is necessary for reproduction. For females, however, the issue becomes more difficult to unravel.

The By-Product Theory

Is the female orgasm simply a by-product of the female’s embryological similarity to males (like why males have nipples that serve no pragmatic function)? This was—and to some extent, still is—a widely believed theory until a 2011 paper titled “Genetic analysis of orgasmic function in twins and siblings does not support the by-product theory of female orgasm” contested this belief.  The study was based on a questionnaire of over 10,000 siblings and found that there was a noteworthy genetic difference of orgasmic functionality between female and males but that there was no statistically significant correlation between siblings of the opposite sex. According to the paper, the data suggest that: “different genetic factors underlie male and female orgasmic function and that selection pressures on male orgasmic function do not act substantively on female orgasmic function.” 

Basically, according to the by-product theory, opposite sex siblings (possessing a similar genetic code) would have similar orgasmic function. The 2011 study, however, showed that there was no significant correlation between opposite sex siblings, refuting the theory. At the time of publication, the debate about this theory still rages within the scientific community.

In addition to the by-product theory, there are several other major hypotheses for why females orgasm. There’s the sperm retention/up suck theory, the pair bonding theory and the mate selection theory.

The Up Suck/Sperm Retention Theory

In the early 1900’s, it was widely believed that the female orgasm was necessary for fertilization, according to the popular up suck theory. In the 1950’s William Masters and Virginia Johnson tested this hypothesis—which postulated that orgasmic contractions sucked semen into the uterus—by applying artificial semen mixed with a radio-opaque substance, which showed on an X-ray, to a cervical cap and had the women orgasm. They found no evidence of up suck.

The theory, near dead, was revived in a different sort of form with the discovery that the hormone oxytocin stimulates uterine contractions during female orgasm, and that these uterine contractions may move semen laterally towards the fallopian tubes. Although the sperm retention theory makes sense, there’s debate over how large of a role these uterine contractions play in fertilization; females, after all, are capable of becoming pregnant without the help of an orgasm.

The Pair Bonding Theory

This theory relies on the rush of feel-good chemicals released during orgasm, such as dopamine and oxytocin. Oxytocin, in particular, is thought to produce feelings of intimacy and bonding while dopamine simulates the brain’s reward center. Basically, this theory postulates that males and females who orgasm together, stay together. And when you’re raising an infant—whether in ancient sub-Saharan Africa or the modern day suburbs—having a partner can make life much easier. The pair bonding theory argues that infants born to paired couples (bonded through frequent orgasms) have a better chance of survival, making the female orgasm evolutionarily significant to the survival of the species.

The Mate Selection Theory

This theory states that the female orgasm was developed as a tool for selecting high-quality mates. In an interview with The Guardian, Tim Spector, a researcher at St. Thomas’s hospital in London, explains it like this: “If a man is considered powerful enough, strong enough, or thoughtful enough in bed or in the cave, then he’s likely to hang around as a long-term partner and be a better bet for bringing up children.” It’s an interesting theory although, like many others, it is still hotly debated.

Although several compelling theories exist for the development of the female orgasm, they have yet to yield a definitive answer.

The orgasm is an extremely complex biological function with interesting evolutionary origin. Understanding the biology behind this beloved event can help us better appreciate the complexities of the human body as well as our own experiences.

Orgasm Facts

Orgasms can help you relax: all of those feel-good hormones we talked about in the article? They can lead to a temporary state of bliss, helping you stay stress-free.

Orgasms may provide pain relief: in a female-focused study, data showed that, on average, an individual’s pain threshold increased after orgasm. NBC news quoted the study, saying: “The pain tolerance threshold and pain detection threshold increased significantly by 74.6 percent and 106.7 percent respectively.” 

Individuals paralyzed below the waist may still experience orgasm. Typically, this is achieved through stimulation of nerves just above where the individual is paralyzed.

According to the Danish National Committee for Pig Production, the up suck theory holds true in pig populations. In a Ted Talk titled: “10 Things You Didn’t Know About Orgasm,” Mary Roach explains that there is a 6 percent increase in the number of piglets produced when the sow is sexually stimulated.