Biology

Mini Review

The Stability and Change unit asked how personality changes over time. The Biology unit asks where the differences between people come from…

The Biology Unit

• How our genes and physical makeup influence our personality.
• How much of the difference between people in a trait is related to their genes, and how researchers can tell.

Poll

Poll Results

Do Children Have Personalities Like Their Parents?

• Parents and their children do have similar personality trait scores, but only a little.
  • Bratko, Butković, and Vukasović Hlupić (2017) combined the personality studies of families.
  • Finding: Parents’ trait scores and their children’s trait scores correlate under .30.
• A correlation that small means that a parent’s extraversion score tells you only a little about the child’s score.
• The question for this unit is why there is any similarity at all.

Why a Family Similarity Cannot Show Its Cause

• Parents give their children their genes AND their environment.
• So when a child’s trait scores are similar to a parent’s, there are two possible causes:
  • The child got the parent’s genes.
  • The child grew up in the parent’s home.
• A study of ordinary families cannot tell these two apart, because every child in it has both.
• To separate them, researchers need people who share genes but not a home, or share a home but not genes.
  • Adoption studies and twin studies provide these people.

Adoption Studies

• An adopted child shares genes with their birth parents but a home with their adoptive parents.
• If adopted children’s personality scores are more like their birth parents’ scores than their adoptive parents’ scores, genes account for the similarity, because the children never lived with their birth parents.
• If adopted children’s scores are more like their adoptive parents’ scores than their birth parents’ scores, the home accounts for the similarity, because the children share no genes with their adoptive parents.

Twin Studies

• Twin study: A research design that compares how alike identical twins are with how alike fraternal twins are.
• Identical twins share all of their genes. Fraternal twins share about half, like any two siblings. But both kinds usually share the same home environment, and are the same age.
• Thus, if differences in a trait are partly due to genes, identical twins’ scores should be more similar than fraternal twins’ scores. If they are not, the two kinds of twins should have scores that are about equally similar.

Heritability

• Heritability: The share of the differences between people in a trait that is due to differences in their genes.
  • Ex: About 40% of the differences in extraversion among adults are due to genetic differences.
  • Ex: Adult height is about 80% heritable.
• A heritability estimate comes from a twin study or an adoption study. It describes one population at one age.

What a Heritability Estimate Means

• Measure the extraversion of 100 adults. Their scores spread out from low to high.
• Some of that spread goes with differences in their genes, and the rest goes with differences in their experiences and in how they were measured.
• A heritability of 40% means that about 40% of the spread in scores across those people goes with differences in their genes. The other 60% goes with everything else.
• The estimate describes the spread across a group. It comes from comparing identical twins with fraternal twins, or adopted children with their birth and adoptive parents.
• Different groups can give different estimates, so the estimate describes one population at one age.

Two Misreadings of a Heritability Estimate

• A heritability of 40% does not mean that 40% of one person’s extraversion comes from genes. Each person’s score comes from both their genes and their experiences, but the estimate does not say how much of one person’s score came from each. It says how much of the spread across people goes with differences in genes.
• A high heritability does not mean a trait cannot change. Height is about 80% heritable. Average height rose across the twentieth century as nutrition improved.

The Environment That Siblings Share

• Shared environment: The common experiences that make people raised in the same family more alike.
  • Ex’s: Family socioeconomic status, neighborhood, parental education, household rules, general parenting style.
  • Ex: Children in a house with a strict curfew learn to be more conscientious about being home on time.

The Environment That Siblings Do Not Share

• Nonshared environment: The unique experiences that make people raised in the same family different from each other.
  • Ex’s: Different peer groups, individual experiences at school, different treatment by parents, hobbies, personal illnesses or accidents.
  • Ex: One sister has a teacher who pushes her into debate, and the other has a coach who pushes her into track.
  • Ex: A broken leg at age nine keeps one twin indoors reading for a year while the other plays outside.

A Shared Home Adds Little to Siblings’ Similarity

• For personality traits, the shared environment accounts for close to none of the differences between people!
  • Adopted siblings who share a home but no genes have trait scores no more similar than two strangers’ scores. If the shared home made siblings similar, these siblings would be more similar than strangers.
  • Identical twins raised in different homes have scores about as similar as identical twins raised together. If the shared home made siblings similar, the twins raised together would be more similar than the twins raised apart.
• Thus, of the differences in personality that are due to the environment, most are due to the nonshared environment.

Why the Nonshared Environment Matters More

• Two sisters grow up in one house. They have the same parents, the same rules, the same neighborhood, and the same family income.
• Those experiences are the same for both sisters, so they cannot make the sisters different from each other.
• The sisters still grow up with different personalities. The experiences that differ between them are the ones that can explain the difference: different friends, different teachers, an illness that only one of them had, or different treatment by the same parents.
• The research finding goes one step further. The shared experiences do not even make the sisters much more similar to each other than to two strangers.
• So the environment shapes personality mainly through the experiences that each person has on their own.

Gene AND Environment Together = Person AND Situation

• People’s personality traits influence the types of environments they find themselves in.
  • Ex: A sociable child is invited to more parties and joins more teams than her quiet sister.
  • Ex: Parents high in openness fill the house with books and take their children to museums.
• Genes and environment are related. Thus, a study that finds a correlation between an environmental variable (school attended, SES, etc.) and a trait cannot rule out genes as an alternative cause, because genes can be related to both.
• This is the Person AND Situation theme. They BOTH influence each other.

How Person AND Situation Fits With Shared and Nonshared Environment

• The shared environment is the same for every child in the home. The parents’ traits shape it, as when parents high in openness fill the house with books, but it does not differ between the siblings.
• The nonshared environment is where a child’s own traits shape the situation. The sociable child’s parties and teams are experiences that her quiet sister does not have.
• So people’s traits help create their own nonshared environment, and the nonshared environment is where most of the environmental differences in personality come from.
• This is one more reason that a correlation between an environment and a trait cannot show that the environment caused the trait. The trait may have come first and led the person into that environment.