Heredity and Hormones

The human body is made up of billions of tiny cells and organ systems that have been created by the genetic material passed down to us from our parents and ancestors. The glands of the endocrine system produces hormones, chemical messengers released into the bloodstream that help regulate bodily activities (Morris & Maisto, 2002, p. 76). These hormones are responsible for the physical changes adolescents endure when they reach puberty; but how do hormones and heredity affect human behavior?

First, let’s look at how human behavior is affected by the endocrine system and the hormones secreted by the glands within this system. The endocrine system works with the nervous system in a constant chemical conversation and plays a key role in helping coordinate and integrate complex psychological reactions (Morris & Maisto, 2002, p. 76). The thyroid gland secretes one hormone, thyroxin, which regulates the body’s metabolic rate and therefore how thin or fat people tend to be (Morris & Maisto, 2002, p. 76). There are many behaviors that are affected by the production of thyroxin. When the thyroid is overactive and too much of this hormone is released, symptoms include over-excitability, reduced attention span, acting out of character, reduced concentration, insomnia, fatigue, agitation, and difficulty focusing on a task (Morris & Maisto, 2002, p. 76). This is not to say that these symptomatic behaviors cannot be exhibited without an overactive thyroid; a person can be agitated by something that happened earlier in the day or fatigued from working hard outside. This is the reason that thyroid symptoms are often misdiagnosed as depression or adjustment problems (Morris & Maisto, 2002, p. 76). The parathyroids, four tiny organs located in the thyroid gland, control and balance the levels of calcium and phosphate, which in turn influence levels of excitability (Morris & Maisto, 2002, p. 76). According to Morris and Maisto (2002), the pineal gland, a pea-sized gland located in the middle of the brain, secretes the hormone melatonin that helps regulate sleep-wake cycles. An imbalance of melatonin produced by the pineal gland may cause insomnia and this could be caused from an increase in thyroxin from an overactive thyroid. The next gland, the pancreas controls the level of sugar in the blood by secreting the hormones insulin and glucagon, which work against one another to maintain a properly balanced blood-sugar level (Morris & Maisto, 2002, p. 77). Without the proper balance of blood-sugar a person could suffer from diabetes or the constant lethargy of hypoglycemia. “The gonads—the testes in males and ovaries in females—secrete hormones that have traditionally been classified as masculine (the androgens) and feminine (the estrogens)” (Morris & Maisto, 2002, p. 77). The male gonad glands produce testosterone, which is the hormone that is increased during the onset of puberty, giving boys body and facial hair and a deeper voice. “Testosterone has long been linked to violence… [,but] when boys and girls have delayed onset of puberty and are therapeutically treated with hormones, girls given estrogen actually become more aggressive than boys given testosterone” (Morris & Maisto, 2002, p. 77). Neither hormone, estrogen or testosterone, causes aggressive behavior on its own, rather it is a complex interaction of chemicals, heredity, and environment that produce certain behaviors. According to Morris and Maisto (2002), estrogen appears to boost women’s performance on tests assessing manual dexterity, verbal skills, and perceptual speed; similar studies with testosterone and elderly men yielded identical results. It makes sense to assume that therapies replacing these hormones would provide the same effect as in pubescent children. The adrenal glands are two glands that each “has two parts: an inner core, called the adrenal medulla, and an outer layer, called the adrenal cortex, [which both] affect the body’s reaction to stress” (Morris & Maisto, 2002, p. 78). When people talk about getting an adrenaline rush, these are the glands responsible for that intense emotional arousal. According to Morris and Maisto (2002), the adrenal glands secrete epinephrine which activates the sympathetic nervous system, increasing heart rate, stopping digestion, enlarging pupils, sending more sugar into the bloodstream, and preparing blood to clot faster, if needed. Norepinephrine, another hormone secreted from the adrenal glands, raises blood pressure by causing blood vessels to constrict and is transported by the bloodstream to the anterior pituitary, where it causes the secretion of still more adrenocorticotropic hormone (ACTH), therefore prolonging the stress response (Morris & Maisto, 2002, p. 78). These two hormones secreted by the adrenal glands are the primary contributors to the “fight or flight” response. The second hormone, norepinephrine, prolongs the response of stress by causing the release of more ACTH to ensure the success of either fighting or fleeing. Finally, the pituitary gland “is often called the ‘master gland’ because of its influential role in regulating other endocrine glands” (Morris & Maisto, 2002, p. 76). From a technological view, the pituitary gland is like the CPU of a computer; it directs the various processes required during a response from the environment. It produces the largest number of different hormones and therefore has the most effects on the body’s functions, such as blood pressure, thirst, contractions during childbirth, milk production, sexual behavior and interest, body growth, the amount of water retained by cells, and other functions as well (Morris & Maisto, 2002, p. 76).

Next, let’s look to what extent our inherited genetic make-up affects our behavior. From one generation to the next, adults pass their traits down to their children through the transmission of genes, the basic unit of inheritance (Morris & Maisto, 2002, p. 76). In a normal human cell there are 23 pairs of chromosomes that carry genes from the father’s sperm to the mother’s egg. According to Morris and Maisto (2002), “the main ingredient of chromosomes is deoxyribonucleic acid (DNA), a complex organic molecule that looks like two chains twisted around each other in a double helix pattern, [and] is the only known molecule that can replicate or reproduce itself” (p. 80). Genes do not provide an accurate direction for a child’s behavior; rather they are maps of potential outcomes. Genes mostly direct the occurrence of inherited physical characteristics. Some behaviors that are inherited are aggressiveness, depression, anxiety, intelligence, smoking, some forms of mental disorders, and alcoholism (Morris & Maisto, 2002, p. 82). Inheriting the genes that produce these behavioral tendencies does not guarantee that these behaviors will occur. “Rather, [genes] affect both the development and operation of the nervous system and the endocrine system, which, in turn, influence the likelihood that a certain behavior will occur under certain circumstances” (Morris & Maisto, 2002, p. 80). Identifying inherited traits does not mean that we are destined to maintain only those behaviors because those traits interact with various environments over the lifespan that influence to what degree we will behave in those ways.

“Much as behavior geneticists try to explain the individual differences in human behavior, evolutionary psychologists try to explain the behavioral traits that people have in common [and they feel] the key to these shared characteristics … is the process of natural selection” (Morris & Maisto, 2002, p. 84). Natural selection states that the strongest of the species survive and reproduce, therefore passing down those strong traits to future generations; the weaker of the species usually die off before reproducing. In the human species, natural selection no longer dictates if the weak survive or die; now our strongest and weakest can flourish in the environment, which means that even maladaptive traits are passed down since the weak are able to live long enough to reproduce. Our technological advancements have made it to where nature no longer selects the traits that are passed down to future generations, and we unknowingly allowed maladaptive traits to spread through our species that we now treat with medications that dull unwanted emotional symptoms. According to Morris and Maisto (2002), “mood exists to regulate investment strategies, so that we spend more time on things that work, and less time on things that do not work [, therefore] … medicating everyone who experiences depression may interfere with a normal defensive strategy that is part of our evolutionary heritage” (p. 86).


Morris, C. G., & Maisto, A. A. (2002). Psychology: An introduction (12th ed.). Upper

Saddle River, NJ: Prentice Hall.

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