Biological Bases of Behavior ❮ 87adreNal glaNdS 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 (norepinephrine), which prepare the body for
“fight or flight,” like the sympathetic nervous system.
PaNCreaS Insulin and glucagon regulate blood sugar that fuels all
behavioral processes.
Imbalances result in diabetes and hypoglycemia, respectively.
ovarIeS aNd TeSTeS Gonads in females and males respectively, necessary for
reproduction and development of secondary sex
characteristics.Genetics and evolutionary psychology
Why do you behave the way you do? To what extent is your behavior determined by your
heredity? To what extent is it determined by your life history or environment? The nature-
nurture controversy deals with the extent to which heredity and the environment each
influence behavior. Evolutionary psychologists study how natural selection favored behav-
iors that contributed to survival and the spread of our ancestors’ genes, and may currently
contribute to our survival into the next generations. Evolutionary psychologists look at uni-
versal behaviors shared by all people. They look at behaviors conserved across related species
to understand how we are adapted to maximize our success in our environments. Charles
Darwin pointed out the similarities of the expressions of emotions in people and other animals,
suggesting that expressions shared across cultures and species are biologically determined.Genetics and Behavior
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. Your genes predispose your behavior. Studies of twins have
been helping to separate the contributions of heredity and environment. Identical twins
are two individuals who share all of the same genes/heredity because they develop from the
same fertilized egg or zygote; they are monozygotic twins. Fraternal twins are siblings
that share about half of the same genes because they develop from two different fertilized
eggs or zygotes; they are dizygotic twins. Heritability is the proportion of variation among
individuals in a population that is due to genetic causes. For the special case of identical
twins, you might want to say that the heritability for traits of identical twins is zero, but
that is not exactly correct. Like evolution, heritability is a concept applied to the popu-
lation rather than the individual. 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 contribution of heredity to behavior. Schizophrenia and general intel-
ligence are more similar in monozygotic twins than dizygotic twins. If monozygotic twins
are separated at birth and raised in different environments (adoption studies), behavioral
differences may reveal the contribution of environment to behavior; similarities may reveal
the contribution of heredity.
Adoption studies assess genetic influence by comparing resemblance of adopted
children to both their adoptive and biological parents. The children must have been
adopted as infants without contact with their biological parents. If the children resemble