Question

Explain the difference between shared and non-shared environments, and give examples (one for each kind of environment) of how they can lead to similarities and differences between a set of siblings.

Answer 1

Shared vs non shared environment:

In order to understand the findings from this research, it is necessary to understand the distinction between two categories of potential environmental influences: shared vs nonshared features of the environment. “Shared” features of the environment are those aspects of an individual’s environment that are necessarily shared with other children in the family. For example, the general personality and general parenting styles and beliefs of the parents, the socio-economic status of the family, the kind of neighborhood that the family lives in, the number of books in the home, and so on, are all features of the environment that would be common to the environment experienced by all children within any particular family.

“Nonshared” features of the environment are any aspects of the environment and any experiences that can be different for different children within the same family. Some of these factors relate to birth order. In a family with three children, the first-born has only younger siblings, the youngest has only older siblings, and the middle child has both a younger and an older sibling. Closely related is the fact that the parents’ parenting styles are likely to have changed from child to child, based upon the parents’ experiences with the older children.

A second source of nonshared features of the environment is pure chance. For example, not all children within the same family will have exactly the same teachers in school, and they definitely will not have exactly the same friends, simply due to random or chance factors.

One other source of nonshared features of the environment, and a source that has been found to be of particular importance, involves features of the environment that are, to a greater or lesser degree, produced by features of the individual him or herself. Parents respond differently to a cranky infant than to a more easy-going infant, and parents naturally adjust their parenting style according to the temperament and behavior of each individual child. As a result, not every child within the same family is treated in the same way. Similarly, an individual who is temperamentally shy will avoid social situations. A temperamentally more sociable individual will spend more time in the company of others. Moreover, although chance factors play some role in determining an individual’s friends, the nature of one’s closest peers will also be affected by the active choices that each individual makes regarding whom they enjoy spending time with.

More generally, we can say that different individuals evoke different responses from others and actively choose different experiences and different kinds of environments for themselves. To the extent that environmental/experiential factors contribute to individual differences in personality, it is these nonshared factors, particularly those that are evoked by characteristics of the individual or actively chosen by the individual, that have been found to be most important.

Example for non shared environment:

Discuss what you think might represent shared environmental influences that contribute to the tendency to smoke. That is, what in the environment might have influenced most people who smoke to start and maintain their smoking habit?

In summary, environments shared by siblings are important in some domains. But, for many personality traits, such as extraversion and neuroticism, shared environments do not seem to matter . Instead, it is the unique environment experienced by each sibling that carries the causal weight.

Twins are also useful in showing the importance of the unique environment (specific to one twin or the other) when studying trait presentation. Changes in the unique environment can stem from an event or occurrence that has only affected one twin. This could range from a head injury or a birth defect that one twin has sustained while the other remains healthy.

The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (which is found for most traits), this implicates that genes play an important role in these traits. By comparing many hundreds of families with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior.