Conventionally, a hormone is defined as a chemical substance released into the bloodstream that regulates tissues at a distance.

Hormones act as both messengers and regulators. Depending on where they act, they can signal through endocrine, paracrine, or autocrine pathways.

Most endocrine hormones are peptides, but some are amines (T3, T4, dopamine, norepinephrine, and epinephrine) and others are steroids (cortisol, aldosterone, DHEA, androstenedione, testosterone, estradiol, progesterone, vitamin D).

Like neurotransmitters, hormones are chemical messengers that must bind to a receptor to deliver their signal. The key difference is distance and timing:

• Neurotransmitters are released very close to their target cells.
• Hormones travel through the bloodstream and can affect any cell that has the right receptor.

Because hormones circulate widely, their effects are often more widespread and usually develop more gradually. They also tend to last longer than the effects of neurotransmitters.

Components of the endocrine system

Endocrine gland	Associated hormones	Functions
Pituitary	Growth hormone, releasing and inhibiting hormones (such as thyroid stimulating hormone)	Regulate growth, regulate hormone release
Thyroid	Thyroxine, triiodothyronine	Regulate metabolism and appetite
Pineal	Melatonin	Regulate some biological rhythms such as sleep cycles
Adrenal	Epinephrine, norepinephrine	Stress response, increase metabolic activities
Pancreas	Insulin, glucagon	Regulate blood sugar levels
Ovaries	Estrogen, progesterone	Mediate sexual motivation and behavior, reproduction
Testes	Androgens, such as testosterone	Mediate sexual motivation and behavior, reproduction

Table adapted from OpenStax

Effects of the endocrine system on behavior

• Changes in mood, circadian rhythm (including the sleep-wake cycle), and libido can be triggered by both hormones and neurotransmitters.

Behavioral genetics

This field examines the relationship between behavioral traits and inherited genes.

• Genes, temperament, and heredity

  • Some aspects of personality come from genetic factors, while environmental factors shape the rest.
  • Temperament is your basic disposition. It can influence later personality development, and many temperament-related traits are inherited. This helps explain why people may resemble their parents in patterns of thinking and behavior.

• Adaptive value of traits and behaviors

  • Traits and behaviors can shift in response to experiences and surroundings. For example, someone might start drinking coffee daily while balancing full-time work and night school.

• Interaction between heredity and environment

  • Genetic and environmental influences work together over time to shape behavior and personality. Behavior doesn’t always match personality. Research suggests that twins raised separately can still show similar behaviors due to shared genetics, even if their personalities differ because of different environments.
  • A problem may have a genetic cause but still require an environmental solution. In phenylketonuria (PKU), a person can’t metabolize the amino acid phenylalanine. Exposure can lead to severe mental impairment, so avoiding foods that contain phenylalanine is essential. Similarly, someone who knows they have a family history of addiction might choose to avoid alcohol or other substances because of a genetic vulnerability.

Influence of genetic and environmental factors on the development of behaviors

• Experience and behavior

  • Personalities and behaviors change in response to life experiences.

• Regulatory genes and behavior

  • Regulatory genes control when, where, and how strongly gene expression occurs. In some cases, this can influence behaviors such as aggression or calmness.

• Epigenetics

  • Epigenetics studies how the same genotype can be expressed in different ways, producing different phenotypes. For example, identical twins share the same genetic information but can develop different traits and disorders because they interact with different environments. This contrasts with the idea that genotype sets fixed limits; instead, it emphasizes how environmental factors can shape gene expression over time.