Why do people act the way they do? There is no simple answer to this question, because social behaviors, like all human characteristics, are influenced by multiple factors. The two most basic influences on social behavior are genes (the chemical instructions that people inherit from their parents’ DNA) and the environment (all other, noninherited factors).
Contrary to a common misconception, genes do not cause behavioral or personality traits, they only influence them. Although genes may be linked to certain traits, it is unlikely that researchers will ever find a single gene that is entirely responsible for most complicated behaviors. First, each gene is not linked to one and only one trait; one gene may influence many different personality characteristics. In addition, many genes work in concert to influence most behaviors, meaning the genetic aspects of a particular trait are the result of small effects over hundreds of individual genes.
Even if all of the genes influencing behavior were discovered, behavior still could not be fully explained nor predicted. This is because people’s environments are just as important in influencing behavior as their inherited genes. Factors such as parenting, schooling, trauma, and the prenatal environment, all play critical roles in the development of social behavior. Even the most highly heritable traits, such as height, are influenced by environmental factors, as demonstrated by malnourished children that are very short despite having tall parents. In this example, environmental factors such as nutritional intake have actually altered the way in which genetically influenced characteristics are expressed.
Therefore, although these two influences are often presented in an either/or fashion, as in the commonly used phrase “nature versus nurture,” evidence suggests that behaviors and other characteristics do not have one clearly identifiable cause. More probable is that both factors are always at work and that for the cause of any given trait researchers should not be asking, “Genes or environment?” but rather, “What is the contribution of each and how do they work together?”
To further understand those factors not due to heredity, the concept of environment can be further broken down. If two individuals experience the same environmental conditions, they are expected to be similar; at the same time, if two people have different experiences, it is assumed that they will behave differently. In behavioral genetics, environmental influences that cause family members to be similar are by definition shared, and those influences that cause family members to be different are nonshared. In the case of twins, the prenatal environment can typically be considered shared, since the developmental conditions experienced are nearly identical. Peer relation-ships provide an example of nonshared environment: Even identical twins growing up in the same household can behave quite dissimilarly, and part of the reason for this can be different peer groups.
As already mentioned, it is now widely held that both nature and nurture simultaneously influence traits and that, to some extent, the environment can influence the expression of genes. It is now clear that the reverse is also true, that genetics influence environment, or at least social relationships. In essence, the two forces operate in such a way that children may create their environment based, at least in part, on genetically influenced characteristics. This is called genotype-environment (GE) correlation, which can be further explained using the terms passive, active, or evocative.
A passive GE correlation is the result of the parents’ genes influencing the child’s environment, which is also correlated with the child’s genes. For example, if a mother and daughter share genes that contribute to their extraverted temperaments, this similarity may contribute to open communication between them. Active and evocative GE correlations reflect situations in which the child’s genetically influenced characteristics influence the behavior of others, including their parents. In active GE correlations, the child purposefully seeks out a particular environment, as in the case of children choosing to participate in extracurricular activities that showcase their natural talent. Evocative (also called reactive) GE correlations result when children elicit responses from others. A child with a difficult temperament, for example, may evoke harshness from a mother that wouldn’t otherwise have behaved as negatively.
Genetic Influences on Parent-Child Relationships
The importance of parenting on the behavior of children is clear from the extensive literature on the topic. Until recently, most studies examining parenting and child and adolescent adjustment assumed that associations between parenting and child behavior were the result of purely environmental influences on the child. The study of genetic influences on parenting, however, has led many developmental and social psychologists who considered themselves “environmentalists” to acknowledge the importance of findings from behavioral genetics. When genetic and environmental contributions of parenting have been studied, significant genetic influences have been demonstrated for both parent and child behavior. In other words, research suggests that genetically influenced characteristics of children and of parents appear to influence the way that parents treat their children.
Twin studies of genetic influences on parenting take two approaches: child-based designs and parent-based designs. In a child-based design, the children are twins or siblings, and the focus is on how genetic influences of the children influence how they are treated by their parents. Parent-based designs examine parents who are twins or siblings and thus, the focus is on the influence of the parents’ genes on how they parent their children.
Parental Warmth and Support
Studies have found that genetic influences on parental warmth and support are best explained by passive GE correlation, meaning that a mother may interact with her adolescent in a positive way, at least in part, because of her own genetically influenced characteristics. This suggests that mothers may treat their children with warmth despite the children’s own characteristics and behaviors. This is further supported by child-based designs that have found that parents are likely to be equally positive to all of their children independent of genetically influenced characteristics of the children. There is also some indication that evocative GE correlation may be operating for parental warmth and support, although these effects are not as pronounced.
Studies have found that evocative GE correlation best explains parental negativity, meaning children evoke negativity from their parents due to their own genetically influenced characteristics. Findings of genetic influences for child-based studies and little or no genetic influences for parent-based designs suggest that parents’ negativity is not influenced by parental genotypes but is influenced by, and is a response to, children’s genetically influenced characteristics. For example, children with difficult temperaments may evoke negativity even from parents with strong genetic influences to be warm and supportive.
The finding that the child’s genetically influenced characteristics evoke negativity from the mother is particularly relevant for potential prevention and intervention strategies because parents can be taught to respond differently to their children. In other words, the implications of this finding are optimistic considering that the prospect of changing elicited negative parental behavior is far less daunting than that of changing genetically influenced negative parenting.
Parent-based studies on parental control, for the most part, show very little genetic influence originating from the parent. This suggests that the level of control parents exert is primarily a response to genetically influenced characteristics of their children. In other words, parental control is primarily evoked by these characteristics of the child (nonpassive GE correlation). For example, children with behavior problems may cause mothers to be more controlling than they would be with more responsible children.
Genetic Influences on Sibling Relationships
Although modest genetic influences have been found for both negative (e.g., rivalry, hostility, and criticism) and positive (e.g., companionship, empathy, and communication) dimensions of the sibling relationship, shared environmental influences are the most important factors in explaining sibling relationships. The importance of shared environmental influences is consistent with the view that sibling relationships are reciprocal in nature and that, more generally, there exists a shared family climate. Support for a shared family climate is also found in studies examining similarities between mother-child and sibling relationships.
Genetic Influences on Peer Relationships
Peer groups are unique in that, unlike families, peers can select each other based on mutual attraction. Research has demonstrated that adolescents, based on their own genetically influenced characteristics, initially seek out friends with whom they share similarities. Moreover, due to socialization, peers grow to be more alike over the course of a continuing friendship.
To date, there are only a few studies examining genetic and environmental influences on peer relationships, and most have focused on the similarities within the peer group rather than on the quality of the peer relationships. Studies examining adolescent peer group characteristics (similarities within the peer group) have found evidence for substantial genetic influences on parent’s perceptions of their adolescents’ peer groups. For adolescents’ own perceptions of their peer groups, genetic influences were less important and nonshared environmental factors were more important. For peer relationship quality, there are several dimensions of friendship moderately linked to genetic influences, including positivity (validation, caring, warmth and support) and, for girls in particular, behavioral and emotional problems. Certain negative aspects, such as conflict, betrayal, and criticism, are associated with the shared environment. Studies of group affiliations have suggested genetic influences on academic aspirations, delinquency, and popularity, although the methods used to draw these conclusions were somewhat problematic. Future research using refined methods will help to clarify the influences on peers and friends.
The basic influences on behavior, therefore, are genes, shared environment, and nonshared environment. Although for any trait one may be more important than the others, all three influences are considered in behavioral genetic studies examining social behaviors. Exploring family members’ genetic relatedness with regard to observable similarities helps researchers estimate the relative ratio of genetic and environmental influences. To this end, researchers use various methods: mainly, family, twin, and adoption study designs.
Twin/Sibling Family Designs
Monozygotic (MZ; identical) twins share 100% of their genes, while dizygotic (DZ; fraternal) twins and full siblings share 50%, on average. Children also share exactly 50% of their genes with each parent. Second-degree relatives such as grandparents, aunts and uncles, and half-siblings are 25% genetically similar, and cousins share 12.5% of their genes, on average. If a trait is largely influenced by genes, the correlation between MZ twins (e.g., twin 1 correlated with twin 2) for that trait should be close to 1.0; for DZ twins, full siblings, and parent—child 0.5; and so on. Accordingly, unrelated children adopted into the same family do not correlate for genetic reasons. Shared and nonshared environmental influences can also be estimated using twin and family designs. Because shared environmental influences are all environmental (non-genetic) factors that make family members similar to one another, such influences would be indicated by correlations that are similar across all family members living in the same household, independent of their genetic relatedness. While this formula is necessary for estimating the relative effects of genes and environment, it becomes problematic in simple family designs (studies in which there is no variation in the genetic relatedness of family members in the same household), whereby the two factors become indistinguishable since individuals that share many genes (e.g., parents and siblings) typically share their environments as well. Finally, nonshared environmental influences are, by definition, all environmental factors that make family members different, including measurement error. The best test of nonshared environmental influences is MZ twin correlations. Because MZ twins reared in the same family share all of their genes and shared environmental influences any correlation less than 1.0 indicates nonshared environment (and measurement error).
Adoption studies are ideal for identifying shared environmental influences. The most common design studies an adopted child reared by genetically unrelated adoptive parents. Any similarity between the child and the adoptive parents must be due to shared environmental factors. Data on the biological parents makes it possible to further parse genetic and shared environmental influences by examining similarities between the adopted child and biological parents and similarities between the adopted child and adoptive parents.
Future Directions in Genetic Influences Research
While studying the relative ratio of genetic influences and environmental influences on behavior and relationships has enhanced our understanding of the social world, researchers are working to use these findings as an avenue to even more specific studies of genetics— gene finding and molecular genetics. Dramatic technological advances allow researchers to analyze specific genes within DNA. Employing statistical analysis (correlation) to associate specific genes with specific behaviors, researchers hope to identify genes that are important in influencing particular behaviors. Using the results of this process (gene finding), researchers then hope to trace, at a molecular level, the pathways from genes to behaviors. Gene finding and molecular genetics studies are currently under way, and their successes would provide unprecedented insights into behavioral processes.
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