looks at hormones, both sexual and nonsexual, as well as other differences between men and women, including different responses and susceptibility to pain and illnesses.
Hormones
Hormones are chemicals released into the blood to activate or regulate bodily functions such as digestion, hunger, stress control, metabolism, growth, lactation, sex drive, circadian rhythm, and reproduction. Most hormones are secreted by specialized glands, like the thyroid gland in the throat, the pituitary gland in the brain, or the pancreas in the upper abdomen.
What's the difference between hormones and neurotransmitters? Hormones travel throughout the body via the bloodstream. Neurotransmitters, which activate and regulate such things as memory, learning, mood, behavior, sleep, pain perception, and sexual urges, travel across synapses in the neurons that make up the brain and nervous system. (Familiar neurotransmitters include serotonin, dopamine, and norepinephrine.)
Scientists have recently realized that in addition to hormones and neurotransmitters, we have neurohormones, which are hormones secreted by or acting on the nervous system.
Hormones are specialized for a wide array of functions, any of which can affect our brains.
Sexual Hormones
There are three basic sexual hormones: androgens (which are mainly male and include testosterone) and estrogen and progesterone (which are mainly female). Both men and women have all three types, but in different amounts, at different levels, and with different receptors (neurons specialized to be sensitive to certain chemicals, including hormones).
Oddly enough, testosterone can turn into estradiol, a type of estrogen. But no matter how complex they are or how they work, sexual hormones make sure that our bodies focus on mating and reproduction that species need to survive. And that can include behaviors that make boys be boys and girls be girls.
Many scientists are studying hormones and behavior in hopes of finding links between the two. For example, studies of the male population show that violent behavior peaks when men have increased testosterone, in the late teens and early twenties. But that doesn't necessarily mean that other behaviors often seen as gender specific are driven by sexual hormones. In other words, it isn't estrogen that makes some people put on makeup, have in-depth discussions about feelings, and shop until they drop, or testosterone that makes others leave their socks on the floor or become fascinated with construction.
The farther away from “nitty-gritty reproductive biology” we get, says Pfaff, the harder it is to clearly link the behavior with sexual hormones.21That's because the human brain is also influenced by so many things, including environment, childhood development, prenatal development, nutrition, toxins, hormones, the conscious mind, and the brain's own structure.
To further complicate things, sexual hormones are released not only as part of internal bodily functions, but also as a response to external experiences. Say you have a date with your spouse that includes dinner, a romantic movie, sweet talk, a candlelit bedroom, and soft fingertips caressing your belly. These external actions can get your estrogen going. Similarly, sexual drive, aggression, and social dominance can make testosterone rise in men. Many studies show a correlation—a coordinated relationship—between testosterone and certain behavior, but it's mostly not known if testosterone causes the behavior or vice versa.
The body can produce hormones in response to external experiences, such as a romantic evening.
“When you examine testosterone levels when males [in a study] are first placed together in a social group,” said Robert Sapolsky, expert on testosterone from Stanford University, “testosterone levels predict nothing about who is going to be aggressive. Behavior drives the hormonal changes, rather than the other way around.”22
So what are these primary sexual hormones that make things so complex?
Testosterone: Testosterone, the primary male hormone, has been shown to boost muscle mass and status-seeking/social-dominance behavior.23 But the testosterone-fueled goal of getting to the top of the heap doesn't always lead to increased aggression; it could generate behaviors such as cooperating with others, if that helps men avoid rejection and, as a result, maintain their social standing.24
Testosterone is produced in the testicles of men, the ovaries of women, and in the adrenal glands of both sexes. In women, testosterone and other androgens can trigger responses similar to those of men, including increased muscle mass and social-dominance behaviors, and it is linked to sex drive. More is being learned about the role of androgens in women, but most scientific attention has been paid to estrogen and progesterone.
Estrogen: Estrogen, the primary female sex hormone, is produced by the ovaries and in small amounts by the liver, breasts, brain, and adrenal glands.25 It has profound effects on just about every part of women's bodies: heart, bones, blood vessels, skin, hair, breasts, mucous membranes, pelvic muscles, and urinary and reproductive tracts.26
In the brain, estrogen affects attention, motor control, pain reception, mood, and memory. In women, estrogen helps regulate the brain's ability to learn and encode memories; testosterone may perform the same function in men, although as mentioned earlier, testosterone makes estrogen.27
Women experience greater fluctuations of estrogen, which can affect women's moods, stress, and learning.28 Low estrogen may increase depression and compulsive behaviors. That's because estrogen regulates emotion in several ways, including:
Increasing serotonin and serotonin receptors, which regulate mood, sleep, and learning,
Modifying the production and effects of endorphins, the “feel-good” neurotransmitter,
Protecting nerves from damage and perhaps stimulating nerve growth.29
Progesterone: This hormone is involved in pregnancy, regulating everything from sex drive, ovulation, the uterine lining, maintenance of pregnancy, and lactation—in short, everything you’d want and need to have a baby.
Much of what we understand about progesterone's effects comes from what happens to our bodies when it's low: infertility, insomnia, sore breasts, weight gain, water retention, vaginal dryness, and decreased sex drive.30 Lowered levels can even cause migraine headaches, depression, panic attacks, abnormal menstrual cycles, and blood-sugar problems.31
Beyond its role as the pregnancy hormone, progesterone has neuro-protective properties and helps with traumatic brain injury by reducing swelling.32 It is positively associated with increased social connection as well.33
Other Hormones
Even the amounts of certain nonsexual hormones (people have over fifty hormones) are different in men and women. What's also different is how sensitive each sex is to them and the disorders each sex can experience because of them. Here's look at three of these hormones.
Oxytocin: When you fall in love, want to cuddle, or are a mom of a baby and want to nurse, oxytocin is at work. Oxytocin (not Oxycontin, the prescribed painkiller) has major