The entire family of internally produced chemicals that resemble opiates in structure and effects.

Theoretical perspective that examines behavioral processes in terms of their adaptive value for a species over the course of many generations.

An electric potential that increases the likelihood that a postsynaptic neuron will fire
action potentials.

People who tend to be interested in the external world of people and things.

Scientific studies in which researchers assess hereditary influence by examining blood relatives to see how much they resemble each other on a specific trait.

A physiological reaction to threat in which the autonomic nervous system mobilizes the organism for attacking (fight) or fleeing (flight) an enemy.

The reproductive success (number of descendants) of an individual organism relative to the average reproductive success of the population.

The largest and most complicated region of the brain, encompassing a variety of structures, including the thalamus, hypothalamus, limbic system, and cerebrum.

Twins that result when two eggs are fertilized simultaneously by different sperm cells, forming two separate zygotes. Also called Dizygotic twins.

DNA segments that serve as the key functional units in hereditary transmission.

The process of determining the location and chemical sequence of specific genes on specific chromosomes.

A person's genetic makeup.

The situation that occurs when two genes in a specific pair are different.

The part of the brain that includes the cerebellum and two structures found in the lower part of the brainstem: the medulla and the pons.

The situation that occurs when two genes in a specific pair are the same.

The chemical substances released by the endocrine glands.

A structure found near the base of the forebrain that is involved in the regulation of basic biological needs.

Twins that emerge from one zygote that splits for unknown reasons. Also called Monozygotic twins.

The sum of an individual's own reproductive success plus the effects the organism has on the reproductive success of related others.

An electric potential that decreases the likelihood that a postsynaptic neuron will fire action potentials.

A relatively durable change in behavior or knowledge that is due to experience.

Destroying a piece of the brain.

A densely connected network of structures roughly located along the border between the cerebral cortex and deeper subcortical areas.

The segment of the brain stem that lies between the hindbrain and the forebrain.

A mating system in which one male and one female mate exclusively, or almost exclusively, with each other.