Environments that are experienced by all siblings in a family and have similar effects on all the members of the family are considered to be shared environments. Family income would exert similar pressures and experiences on all the siblings in a family. Behavioral genetics studies siblings (often twins) in order to observe the relationship between genetic and environmental factors. They do this by isolating environmental variables. For instance, if psychologists know whether or not siblings possessed a shared environment, then they can determine what traits are most influenced by specific family characteristics and what traits are most influenced by environmental factors outside the home.

Environments that are unique to individuals or those that are experienced differently by each sibling will have varying effects on each sibling. Being treated differently by their parents would create different environments and have different impacts on the behavior of siblings in a family. Non-shared experiences can influence many factors of a child's personality and differentiate them from their siblings. Anything that a person experiences uniquely qualifies as a non-shared environment.

Chromosomes are made up of DNA and store the genetic material of an individual. Chromosomes are found in the nucleus of human cells and contain all of our DNA. Genes are found in specific loci (locations) on chromosomes. These terms are commonly used in many areas of psychology that use genetics to describe certain behaviors or outcomes.

"Alleles" are the different versions of a gene. Alleles can have varying patterns of inheritance like dominant alleles and recessive alleles. They can be found at the same location on a specific chromosome.

Humans have a total of 23 chromosomes. 22 of them are autosomes, or non-sex chromosomes, and 1 pair are sex chromosomes. Autosomes are chromosomes that control traits like skin and hair color. Sex chromosomes determine things like gender and different gender characteristics.

"Crossing over" occurs in prophase I of meiosis. It involves the swapping of genetic material between homologous chromosomes. This creates genetic variability in individuals. Crossing over is extremely important because without genetic variablity humans would be very vunerable to different diseases or dysfunctions.

A dominant pattern of inheritance describes traits that are expressed over other traits. For example, brown eyes are dominant to blue eyes. It only takes one dominant allele for a trait to be expressed. It takes two recessive alleles for a recessive trait to be expressed.

"Sex-linked" traits are passed down by the X or Y chromosomes of the parents. A female gets the chromosomes XX and a male gets XY. This is especially important in fields like developmental psychology where psychologists need to understand genetic disorders that can effect the cognitive development of a child. Sex linked traits have patterns of inheritance that make them unique from autosomal traits. For example, if a trait is Y linked, then a father will pass it down to all of his sons because he passes down his Y chromosome to each one of them. on the other hand, his daughters would not have the trait because he only passes down an X chromosome to each of them.

Developmental psychologists study humans of all ages to observe how different stages of life (e.g., infants, adolescents) react to various stimuli. A significant proportion of research in developmental psychology focuses on children, seeing as childhood sees the greatest amount of change.

Erik Erikson was a famous psychologist who proposed eight psychosocial stages that every human being experiences during his or her lifetime. Erikson believed that adolescents experience confusion over which roles they wish to fulfill (e.g., rebel, good student, class clown). To successfully conquer this stage, adolescents must merge their various roles of choice into a complete identity. This conflict is known as identity vs. role confusion.

A general overview of Erikson's stages is given below:

Trust vs. mistrust, age 0-2 years

Autonomy vs. shame and doubt, age 2-4 years

Initiative vs. guilt, age 4-5 years

Industry vs. inferiority, age 5-12 years

Identity vs. role confusion, age 12-19 years

Intimacy vs. isolation, young adulthood

Generativity vs. stagnation, middle adulthood

Ego integrity vs. despair, late adulthood