Biological Bases of Behavior ❮ 95• Hypothalamus—portion of brain part that acts as endocrine gland and produces
hormones that stimulate (releasing factors) or inhibit secretion of hormones by the
pituitary.• Pituitary gland (sometimes called “master gland”)—endocrine gland in brain that
produces stimulating hormones, which promote secretion by other glands including TSH
(thyroid-stimulating hormone); ACTH (adrenocorticotropic hormone), which stimulates
the adrenal glands; FSH (follicle stimulating hormone), which stimulates egg or sperm
production; ADH (antidiuretic hormone) to help retain water in your body; and HGH
(human growth hormone).• Thyroid gland—endocrine gland in neck that produces thyroxine, which stimulates
and maintains metabolic activities.• Parathyroids—endocrine glands in neck that produce parathyroid hormone, which
helps maintain calcium ion level in blood necessary for normal functioning of neurons.• Adrenal glands—endocrine glands atop kidneys. Adrenal cortex—the outer layer—
produces steroid hormones such as cortisol, which is a stress hormone. Adrenal
medulla—the core—secretes adrenaline (epinephrine) and noradrenaline (norepi-
nephrine), which prepare the body for “fight or flight” like the sympathetic nervous
system.• Pancreas—gland near stomach that secretes the hormones insulin and glucagon,
which regulate blood sugar that fuels all behavioral processes. Imbalances result in
diabetes and hypoglycemia.• Ovaries and testes—gonads in females and males, respectively, that produce hor-
mones necessary for reproduction and development of secondary sex characteristics.Evolutionary psychologists—study how Charles Darwin’s theory of natural selection
favored behaviors that contributed to survival and spread of our ancestors’ genes; evolu-
tionary psychologists look at universal behaviors shared by all people.
Behavioral geneticists—study the role played by our genes and our environment in
mental ability, emotional stability, temperament, personality, interests, etc.; they look at
the causes of our individual differences.
Zygote—fertilized egg.
Studies of twins help separate the contributions of heredity from environment.
• Identical twins—also called monozygotic twins; two individuals who share all of the
same genes/heredity because they develop from the same zygote.• Fraternal twins—also called dizygotic twins; siblings that share about half of the
same genes because they develop from two different zygotes.Heritability—the proportion of variation among individuals in a population that is due
to genetic causes.
When twins grow up in the same environment, the extent to which behaviors of
monozygotic twins are behaviorally more similar than dizygotic twins reveals the contri-
bution of heredity to behavior.
If monozygotic twins are separated at birth and raised in different environments (adop-
tion studies), behavioral differences may reveal the contribution of environment to
behavior; similarities reveal the contribution of heredity.