The Genetic Basis of Behavior & Evolution (College Board AP® Psychology): Study Guide

The genetic basis of behavior

• The biological basis of behavior refers to the idea that behavior and mental processes are shaped, at least in part, by heredity

• Heredity refers to the predisposed genetic characteristics passed down from one generation to the next

  • These genes influence physical, behavioral, and mental traits and processes, e.g., having a genetic predisposition toward anxiety or depression

• Genes are the basic biological units responsible for carrying information about traits between successive generations

  • Many physical and psychological characteristics are at least partially inherited

• The term heritability refers to the degree of variance among individuals for a specific trait that can be attributed to genetic differences

  • It does not mean a trait is entirely determined by genes, but rather that genes account for some of the differences observed between people

• Importantly, no single gene is thought to be responsible for complex psychological traits or disorders

  • It is more likely that different combinations of genes make individuals more or less vulnerable to particular behaviors or conditions

• Psychologists investigate the genetic basis of behavior using three main non-experimental research methods:

  • Twin studies

  • Family studies

  • Adoption studies

Twin studies

• Twin studies compare monozygotic (MZ) twins with dizygotic (DZ) twins

  • MZ twins are identical twins who develop from a single fertilized egg and share 100% of their DNA

  • DZ twins are fraternal twins who develop from two separate eggs and share approximately 50% of their DNA

    • DZ twins are no more genetically similar than any other sibling pair

• Because MZ twins share all of their genetic material, any greater similarity between MZ twins compared to DZ twins on a specific trait is taken as evidence of a genetic influence on that trait

• If a trait were entirely determined by genetics, 100% of MZ twin pairs would share it

  • This is rarely found - twin studies consistently demonstrate that both heredity and environment contribute to behavior and mental processes

Research support

• Joseph (2004) analyzed data from all schizophrenia twin studies conducted prior to 2001

  • MZ twins showed a much higher rate of similarity (40.4%) than DZ twins (7.4%)

  • This suggests a substantial genetic contribution to the disorder

  • This finding also demonstrates that environment plays a role since MZ twin pairs did not show 100% agreement

• Bouchard et al. (1990) conducted a large-scale twin study examining heritability of IQ

  • MZ twins reared together showed a correlation of +0.86, suggesting a strong genetic contribution to intelligence

  • The finding shows that environment also plays as role in intelligence as the correlation is not a perfect +1.0

Limitations

• Twin studies are non-experimental

  • Researchers cannot randomly assign participants to be MZ or DZ twins, meaning causation cannot be established

• MZ twins tend to be treated more similarly by parents, teachers, and peers than DZ twins

  • E.g. they may dress alike, share the same friend groups and join the same extracurricular activities

  • This means that their greater environmental similarity, not just their greater genetic similarity, may partly account for their higher rates of agreement on traits and behaviors

Family studies

• Family studies examine whether specific traits or psychological disorders cluster within biological families, on the basis that family members share a proportion of their genetic material

• Researchers compare rates of a trait in first-degree relatives (parents, siblings, children) versus more distant relatives or the general population

  • First-degree relatives share approximately 50% of DNA

• If a trait has a genetic basis, first-degree relatives of an affected individual should show higher rates of that trait than the general population

  • Rates should decrease as genetic relatedness decreases

Research support

• Gottesman (1991) compiled family study data on schizophrenia, illustrating clearly how risk increases with genetic relatedness:

  • The general population has a 1% risk

  • First cousins have approximately a 2% risk

  • Siblings approximately a 9% risk

  • Children with one affected parent approximately a 13% risk

  • Children with two affected parents approximately a 46% risk

• This graduated pattern of risk, tracking closely with degree of genetic relatedness, provides compelling evidence for a hereditary contribution to behavior and mental processes

Limitations

• Family members also share environments

  • E.g. homes, schools, economic circumstances, and communication patterns, which makes it difficult to separate genetic from environmental influences

  • Therefore, family studies alone cannot establish that a trait is inherited rather than environmentally transmitted

• The sample of families studied may not be representative of the wider population

  • Families who come to the attention of researchers are often those already known to clinical or medical services, meaning they may represent more severe cases

  • This could mean findings cannot be generalized to the wider population

Adoption studies

• Adoption studies examine individuals who have been raised by non-biological parents

  • Adoption studies allow researchers to separate the effects of heredity from environment more clearly than twin or family studies alone

• If an adopted individual shows greater similarity to their biological family than their adoptive family on a specific trait, this is taken as evidence for a genetic influence on that trait

  • If an adopted individual shows greater similarity to their adoptive family, this points to the importance of environmental factors

Research support

• Tienari et al. (2000) studied 164 adoptees whose biological mothers had been diagnosed with schizophrenia

  • 6.7% of these adoptees also received a diagnosis of schizophrenia, compared to only 2% of 197 control adoptees born to mothers without schizophrenia

  • This suggests a genetic contribution to the disorder that persisted even when children were raised in different environments

Limitations

• Adoption studies assume that the adoptive families of children with a biological family history of a disorder are no different from other adoptive families

  • In practice, potential adoptive parents are often informed of a child's biological background before adopting

  • This means that families who choose to adopt such children may not be representative, e.g. they may have personal experience of the disorder themselves

  • This is a potential confounding variable that complicates interpretation of findings

• Adoption studies are non-experimental

  • Researchers cannot randomly assign children to biological or adoptive families

  • Additionally, adopted children may be selectively placed in environments similar to their biological family's background

  • This means that similarities between adopted children and their biological families may partly reflect environmental similarity rather than genetic inheritance

The evolutionary perspective

• The evolutionary perspective explores how natural selection affects the expression of behavior and mental processes, in order to increase survival and reproductive success

• Natural selection can be defined as:

  • The process by which traits that improve an organism's chances of survival and reproduction are passed on to future generations, while traits that reduce survival and reproduction gradually disappear from the population

• Behaviors that are adaptive are more likely to be inherited and maintained across generations

  • This is why certain behavioral tendencies appear to be widespread across human populations

• Behaviours that evolutionary psychologists argue have been shaped by natural selection include:

  • Aggression

    • Aggression may have conferred survival advantages in competition for resources and protection against threats from predators and rival groups

  • Memory

    • The ability to recall the location of food sources or environmental dangers would have been critical for survival in ancestral environments

  • Language

    • Language enables communication, cooperation, and coordination within social groups, enhancing collective survival

  • Mate selection

    • Tendencies to select mates based on indicators of health and genetic fitness may have increased the likelihood of producing healthy offspring, thereby increasing reproductive success

• The evolutionary perspective proposes that to fully understand a behavior, we should ask not just how it works, but why it exists

  • This is known as asking about the ultimate cause of a behavior

Eugenics

• Eugenics is a discredited branch of thinking that misapplied principles of the evolutionary perspective to argue that some human groups were genetically superior to others

  • This was used to justify discriminatory policies including:

    • forced sterilization of people deemed "unfit" to reproduce, including those with mental illness, disabilities, or from certain ethnic groups

    • racial segregation and anti-immigration laws in the United States in the early 20th century, based on claims of genetic inferiority

    • the Nazi regime's genocide programs in the 1930s–40s, which used eugenic ideology to justify the systematic murder of millions of people

• Eugenics is scientifically unfounded — there is no evidence that any human group is genetically superior to another

• It serves as a stark example of how evolutionary ideas can be misapplied in ways that discriminate against and cause devastating harm to others