Human Sexual Behavior - part 1.

Our goal is to look at a behavior. First to consider its fixed action patterns. Then step back and look at origins for this behavior, i.e. what happened in the brain & environment, its stimulus. All of these at different time scales - 1 second ago, 1 minute and up to millions of years.

There is a proximal mechanism for sexual behavior - it feels good (driven by sensory stimuli, feedback and sensations). In species with only reproductive sex, pleasure depends on ovulation. In humans it is different (but it is still easier when estrogen levels are higher).

Many species have similar fixed action patterns, e.g. lordosis reflex, pelvic thrusting, orgasm, ejaculation. At the same time there are differences across species.

Fixed action patterns create chains of behavior: 1) Fixed action pattern of individual A is a releasing stimuli for individual B. 2) Resulting fixed action pattern of B is a releasing stimuli for individual A. 3) Loop.

This creates species specificity.

Thus, ethological approach is essential when studying sexual behavior. E.g. studying female rat sexual behavior in a cage is like studying dolphin swimming in a bathtub. In case of a female rat, there was a large belief that the role of a female rat was very passive. However, this was caused by the cage and lack of space to run around.

Studying in people - anonymous surveys or something like randomized response.


Libido was used before, but it is now outdated. Instead in the literature one uses:

  • attractivity (how attractive an individual is)
  • proceptivity (active behaviors being carried out in response to being attracted to that individual)
  • receptivity (how receptive a given individual to the interest of others)

The flow is the following. Because of attractiveness of individual A, individual B began proceptive behaviors, which did or did not prove to meet receptive fixed action patterns of A in response.

Female orgasm

It is present not only in humans.

Fertility does not depend on female orgasm. Thus, it is not necessary to increase number copies of your genes. However, it may facilitate fertilization:

  • orgasm increases vaginal secretion and vaginal fluid increases sperm motility (indirect evidence)
  • orgasm exhausts - a female lays horizontally - sperm does not have to swim against gravity (leading model)
  • it feels good, thus, positive reinforcement.

In the end, no connection between fertility and female propensity towards orgasm was observed. There is a certain degree of heritability of propensity towards orgasm in females. Likelihood of female orgasm depends more on a female than on a male.

Clitoral orgasm

Can be spandrel - baggage from male ejaculation. E.g. nipples are spandrels in men (women have to go through lactation, it is “expensive” to evolve men without nipples).

Sexual behavior unique to humans

There are many examples of behavior which are falsely believed to be unique to humans:

  • Non-reproductive sex (bonobo, chimps, dolphins)
  • Foreplay (bonobo, chimps)
  • Homosexuality
  • Quest for variety (bonobo)
  • Confusion with violence (psychopathological)
  • Masturbation
  • Fantasy (baboons)
  • Cheating

Unique behaviors:

  • Egalitarian sex
  • Private setting
  • Marriage. > 90% of human end up in stable relationship even across polygamous cultures. However, there is a lot of social monogamy, but less of sexual monogamy. In Western countries 10-40% of all children in marriage are not from marriage husband. Note: I tried to find a proof (since this looked like a lot) and there is at least one article mentioning 1-5% range instead, which feels more plausible to me. The number is hard to estimate, since fathers who doubt their paternity are more likely to do the test. Thus, the rate can reach 10-40% if you just consider rate at paternity testing laboratories (which is not a good proxy for general population).

  • Romance. Couple centuries old concept. The notion that it should last through relationship is only 30-50 years old.

  • Somewhat boring - no sex changes, not hermaphrodites.

Average duration of a marriage is 2-4 years, which is close to average inter-birth period and for how long children critically depend on parents. Thus, humans tend to be serial monogamists.


Limbic system is responsible for fixed action patterns. Different areas are relevant for different genders. In case of female:

  1. Subsection of hypothalamus (HT). Here is a ventral medial (VMH) - hot spot for receptors for estrogen and progesterone.
  2. Midbrain
  3. Spinal pathways


  1. Medial preoptic area in HT responsible for performing. Here are receptors for testosterone & androgen.
  2. Amygdala (sexual motivation). Thus, males confuse sexuality with aggression.
  3. Autonomic nervous system for erection (clitoral erections in females work like that too).

Types of erection mechanisms

It can be vascular or muscular. Vascular is based on blood flowing in and not flowing out and lasts longer. Muscular is based on a muscle and occurs faster.

Dimorphism across genders

There is a similar orgasm physiology across sexes, but recovery time is different (slower for females). E.g. for males too fast transition from parasympathetic system to sympathetic leads to premature ejaculation. For females a failure of transition leads to inability to reach orgasm.

Brain also has areas, which differ across sexes, e.g. INAH cluster in hypothalamus. Size 2:1 for men:women. One can almost identify gender by size of this area.

Hormonal response

In females having sex leads to increase of progesterone derived hormones (i.e. reinforcing pleasure). Also without adrenal glands sexual motivation and arousal goes down in females (i.e. androgens play a role). Oxytocin is responsible for forming attachments and pair bonds. Higher level of oxytocin leads to more trust and cooperation (even when sprayed through nose). Neuromarketing is very interested in such topics.

Normally oxytocin is used for nursing and milk. As a result, monogamous attachment is a descendant of neurobiology of mother-offspring attachment.

For males - testosterone goes up after sex. There is no evidence for higher testosterone causing more sex activity.

Vasopressin is responsible for forming pair bonds. Monogamous species have receptors for vasopressin on dopamine releasing neurons. More such receptors, the faster pair bonds can be produced. Similar in monkeys. Humans are as usual somewhere in between on this mono-poly scale, but there are genetic variations.

However, the mechanism is not fully understood, e.g. bonobos have monogamous receptors, but they are hyper polygamous.

Sexual orientation

As it was mentioned above, INAH area differs across genders. However, homosexual men have size of this area similar to hetero women. This was discovered in a study by Simon Levay and has been replicated. However, the brains were from AID victims.

There is also hypothalamus difference. Dr. Schwab published this and even got death treats from homosexual community, since this was perceived as pathology. Levay was gay and his finding was met much better. Partially since its connection to gender and sexuality was better understood. This hypothalamus area just happened to be close to the area Levay discovered (i.e. its size difference is caused by proximity to and INAH size difference).

Overall, INAH study was perceived positively, since it was showing that orientation can not be chosen, but it is just a biology instead. There was even a t-shirt “the only small thing about me is my sexually dimorphic nucleus” (i.e. INAH).

There is also a difference in ratio of length of 2nd finger vs 4th. Homosexual men on average tend to have this ratio close to straight women.

Otoacoustic reflex. If you close your ears, you hear oscillations and their rate differs by sex. Homosexual men have frequency more typical of straight women.

For homosexuality in women there are fewer studies. The 2nd/4th finger ratio is the same (i.e. homosexual women have ratio of straight men) as well as otoacoustic reflex.


Used to be a psychiatric disorder (as well as homosexuality).

There is a gender difference in size of bed nucleus of the stria terminals (reliable predictor for gender).

Transsexuals have the size of their self identified gender (i.e. not their physical gender). This holds even for people who didn’t do gender change. Moreover when men with testicular cancer are treated with feminine hormones, this does not change size of this area.

60% of victims of penile cancer have phantom penile sensations. Transgender people, who changed their gender from male to female get no phantom penile sensations.

In other words, transsexuals got a wrong gender body.


Neurobiology of pleasure, anticipation and reward is common. Role of dopamine is identical across genders and is huge. Loss of sexual interest is one of defining symptoms of clinical depression. Mesolimbic dopamine system is central to reinforcing aspects of sexual behavior. It is about anticipation of the reward and fueling of behaviors needed to achieve reward.

In males this system activates, if one finds another person attractive and there is eye contact or the other person is not attractive and there is no eye contact. Same holds for homosexual men looking at pictures of other men. Sexual orientation in this mechanism is just like having other gender areas.

Dopamine is related to uncertainty. E.g. if a monkey gets reward only in 50% of cases, the dopamine level will be even higher than when 100% reward. This is called intermittent reinforcement. If the rate is changed to 25% or 75%, the level falls, but still higher than 100% (i.e. corresponds to level of uncertainty). Thus, the goal of Las Vegas is to convince people that there is 50% change, when there is not.

Joke: relationship is the price you pay for anticipation of it.


There are receptor subtypes D1 and D2, which are relevant for sex behavior. In monogamous rodents, when bonding happens level of D2 goes down and D1 increases. If D2 falls before mating, no pair bond happens. If decrease of D2 or increase of D1 is prevented, the individuals will bond with someone else.

D2 - rewarding anticipatory aspects of pair bonding D1 - pleasure of monogamous features of pair bond.

In human ratio of D2/D1 predicts stability of relationships.

There was a brain imaging study which considered couples after 2.5 weeks and after 5 years. In the first case, just seeing their beloved one activated this dopamine system. In the second case, anterior cingulate was activated instead (i.e. empathy, comfort)

Brain areas

Frontal cortex is responsible for context of sexual behavior (i.e. how to be appropriate). This does not change fixed action patterns, but their context instead. Damage to this area can lead e.g. to a rape in a nursing home.


Non human primates (as well as humans) are very visual (males usually more). As a result, they get more dopamine activation due to visual stimuli.

Sometimes some tactile receptors activate more dopamine, this explains erogenous zones.

Responsiveness to stimuli also depends on hormone levels. In women, there is more sensitivity during ovulation (especially genitals). Overall skin is more sensitive to sexually arousing touch during ovulation. In men testosterone is required.


There are olfactory cues, i.e. pheromones.

In both sexes right hormones are required to generate pheromones. Men with low testosterone don’t generate pheromones, neither do women without ovaries.

Pheromones - breakdown products of sex hormones. Neuromarketing is also interested in this area.

Perfumes are classically made of sweat of various animals (typically male). E.g. Chanel #5 was based on sweat of whipped male Abyssinian cats. Currently it is more synthetic and made of androgens. The sweat is from males, because majority of perfumes are not bought by men.

Pheromones convey a lot of info, e.g. species, gender, whether castrated or not, health, terrified or not, family-related to you.

To perceive pheromones, the receiver needs right hormones, e.g. castrated men don’t distinguish sweat of men and women.

Females identify gender via pheromones even better when they ovulate. Homosexual men are even better at detecting other homosexual men smell.