Shared Environments, Different Personalities

From the lecture series: Understanding the Mysteries of Human Behavior

By Mark Leary, Ph.D., Duke University

Both genetics, and the environment play an important role in determining human personality, but researchers have been surprised by exactly how the environment affects us. Professor Mark Leary explains why.

(Image: Stephen Denness/Shutterstock)

Two Siblings, Different Experiences

Siblings offer the best example of a “shared environment.” We tend to overestimate how similar the environment is for children who grow up in the same family. As they grow older, children spend more and more of their time in situations that don’t involve their siblings. Some of the differences are obvious—having different teachers from your siblings and sitting in different classes all day. You and your siblings each tended to play with your specific friends. If you played sports, you were on different teams, which featured different coaches, teammates, and win-loss records.

Birth order and the configuration of a family also matters. The firstborn in a family will spend a year or more as an only child, while the younger sibling always has an older brother or sister from day one. The lives of children in the same home are much more different than we often assume. As a result, many more unshared experiences cause their personalities to be different.

This is a transcript from the video series Understanding the Mysteries of Human Behavior. Watch it now, on The Great Courses Plus.

Another reason that shared environments exert relatively little influence on people’s personalities is that the same environment or the same experience might be perceived uniquely by different children. Divorce is a good example. For children living at home, the divorce of their parents is a shared event, and we might expect it would have a similar effect on their personalities. But an objective environmental event, such as divorce, may be perceived differently by each child. One child may be distraught that their father is moving out, and another child may be mildly pleased because they think their father is mean.

Learn more about psychological development in childhood

Same Event, Different Experience

Although the children appear to have experienced the same objective event, the event isn’t the same for all of them. If the children perceive the event differently, it may affect them in distinct ways. Is divorce a shared or an unshared event? It depends on whether we’re talking about the objective event, which is shared, or the idiosyncratic impact of the event, which is non-shared.

Little girl crying on the beach
Siblings sharing a same event, such as a beach vacation, may experience it differently.(Image: KuAlla/Shutterstock)

If you have brothers or sisters, you may disagree with them about whether certain shared experiences were good or bad. Maybe you loved the small town you grew up in, but they found it stifling. Perhaps they enjoyed the family vacations to the beach, and you preferred the ones to the mountains.

Looking at family experiences from the outside, an observer might think that you and your siblings shared many experiences. The truth is your subjective experience of living in the small town or going to the beach was much different than that of your sibling.

Genes Affect the Experience

To complicate matters, certain genes influence behavior only when a person experiences a particular environmental event.

Serotonin neurotransmitter molecule
 Serotonin contributes to feelings of well-being and happiness. (Image: ghost design/Shutterstock)

There are two forms of a gene that is related to the neurotransmitter serotonin. Under normal circumstances, people with these two different forms of that gene, called alleles, don’t act differently. When they experience highly stressful life events, people with one form of the gene are likely to become depressed, whereas those with the other form of the gene don’t become depressed.

Learn more about nature, nurture, and human behavior

Imagine that we have two siblings, each with a different form of this gene. Also, imagine that the family experiences some very stressful event—the parents become unemployed, there’s a death in the family, or the parents start fighting. Differences in this gene may lead the two children to respond very differently to the same family event.

Researchers have uncovered a similar effect where mistreated children are more likely to engage in antisocial and delinquent behavior if they have a particular gene. For instance, two siblings who are treated badly by their parents may have different reactions because they have different genes.

Researchers have no idea how many genes have environment-dependent, but behavioral geneticists assume these kinds of gene-environment interactions are common.

The Effect of Shared Environments

A few areas in which effects of the shared environment have been found tend to be in attitudes and beliefs, rather than personality traits. Children do tend to adopt the belief systems of their parents to some extent, and even if they reject their parents’ beliefs, later on, those beliefs still affect them at times.

The fact that the shared family environment has little effect on the development of people’s personalities has led some people—both researchers and popular writers—to conclude that if the shared family environment is unrelated to children’s personalities, then the parents’ influence does not make a difference in how their children turn out. From what has been discussed, this is not a valid conclusion.

The research on shared and unshared environments doesn’t show that parents don’t have any effect on their kids. Rather, it shows that parents don’t have the same effect on all of their children. The fact is that parents don’t treat all of their children the same way. Many parents try, thinking that they should treat all of their children alike, but they can’t. Each child is already different at the time they’re born. Based on their genetic makeup, they have different traits, needs, and ways of reacting that good parents respond to differently.

Learn more about why siblings can be so different

Parents will naturally respond differently to the shy child who is interested in books than they respond to the assertive, active child who gravitates toward sports. To treat these children the same would be a great disservice to both of them and a sign of bad parenting.

Parents teaching their children to paint
Parents don’t have the same effect on all of their children. Children may respond differently to precisely the same parenting.

Parents don’t have the same effect on all of their children. Children may respond differently to precisely the same parenting.

The implication, however, is that the parents create different family environments for different children. They may have a great impact on both of their kids, but their behavior as parents will often cause differences rather than similarities in the children’s personalities. On top of that, children may respond differently to precisely the same parenting. The free-spirited child may resist parental control that a docile or anxious child may appreciate.

The parents are affecting their kids, but the children are reacting in distinct ways. This is another reason why children from the same family can be so different. The take-home message is although we are often surprised when children from the same family turn out differently, once we understand the processes that are involved, human personality is not as much of a mystery as it first appears.

Common Questions About Shared Environments

Q: What is shared environmental factors?

In psychology studies, shared environmental factors refer to common experiences of siblings living in the same household. These commonalities could include household income, the family’s living situation, the dynamics between the parents (or if the parents are divorced), and even foods that are consumed by both siblings. The aim of these studies is to see whether such similarities would lead to personality similarities between the siblings.

Q: What is the equal environments assumption?

The equal environments assumption poses that if siblings are raised in similar environments, their behavior will also share similarities.

Q: What is the difference between shared and non-shared environments?

The difference between shared and non-shared environments is that shared environments refer to common experiences between siblings such as living conditions, while non-shared refers to separate experiences such as friends, teachers, etc. which each sibling has independent of the other.

Q: Why is equal assumption important?

Equal assumption is important because it provides the foundation from which to test whether or not shared environments do indeed significantly influence sibling behavior.

This article was updated on December 24, 2019

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