Both twin studies and adoption studies are used to examine the effects of nature and nurture on behavior, where nature means the biological or genetic causes of behavior and nature means environmental causes of behavior (Psychology Wizard). Most often, twin studies are focused on examining the role of nature and adoption studies the role of nurture or the extent to which children who are not raised by their biological parents are similar or different to their adoptive parents. The assumption is that if these children share the adoptive family’s traits, the traits are a product of nurture, whereas if the child grows up with traits that resemble his or her biological parents, it points to nature. 

Twin Studies

Twins are rare and the incidence differs according to geographical location. The chances of a mother giving birth to twins in the UK is only 1.5%, but in Nigeria, it is 5%, and in Japan just 0.3% if the mother is under 25 (Psychology Wizard). There are two types of twins: Identical twins share the same sex and 100% of their genetic information (Medicine Encyclopedia) and are even more rare ‒ just 0.5% in the UK (Psychology Wizard). Fraternal twins share about half their genes or the same amount of genes as a brother or sister and may be of different sexes. 

In twin studies, the focus is on comparing identical and fraternal twins because it can reveal the influence of nature on behavior. In twin studies, the concordance rates of identical and fraternal twins are compared. Concordance, usually expressed as a percentage, reflects that both twins show the same behaviors (Psychology Wizard). If both identical and fraternal twins behave in the same way, it suggests nature plays a less important role than nurture, whereas if they behave differently, it suggests nature plays a more important role than nurture or the environment.

For example, twin studies comparing identical and fraternal twins show that fraternal twins of alcoholics have about risk of becoming alcoholics as full siblings whereas identical twins have significantly higher risk (Hong V. Nguyen), and some studies find the risk may be stronger for men than women (Susan Nolen-Hoeksema).

Adoption Studies

Some 5330 children were adopted in the UK in 2015 (Psychology Wizard). Adoption studies generally use correlational statistics, but it is best to remember that a correlation does not necessarily indicate a causal relationship because a number of extraneous and confounding variables my interfere with the strength of a correlation. With respect to adoption, separation from the biological parents due to tragedy or infants being separated from their mothers during a critical period may interfere affect the correlation (Psychology Wizard). Adopted children may thus not accurately represent children in general. In addition, adopted children may have been fostered by other families before being adopted, and agencies attempt to match children with adoptive parents with respect to race, ethnicity, class, and even language, thus increasing the similarities that may be found.

So, are we a product of nature or nurture: 

Twin studies are important for establishing if certain medical and medical conditions are inherited, for example, schizophrenia, but concordance is rarely 100%, suggesting that genetics or nature may influence but not cause behavior. The onset of schizophrenia must be triggered by the environment. However, behavioral genes have been shown to exist with respect to fraternal twins. When fraternal twins are reared apart, they show the same similarities in behavior and responses as if they have been reared together (Sarah Mae Sincero). Moreover, a representative sample of 973 twin pairs to test the heritable differences in subjective well-being were found to be fully accounted for by the genetic model of the Five-Factor Model’s personality domains.

Table of the differences between adoption and twin studies

Summary

While both adoption and twin studies are natural experiments, in so much as they examine variables that occur naturally rather than manipulate variables and are used to determine to the extent behavior is a product of nature (genes) or nurture (environment), twin studies are superior to adoption studies because adopted children do not share 100% of their genes with either their biological or adoptive parents. Adoption studies, however, are helpful for studying the effects of nurture because a researcher can be sure that biological parents contribute little or nothing other than their genes.

FAQ

How are adoption studies used to study nature Vs Nurture?

Adoption studies attempt to establish correlations between the behaviors and traits of adopted children and their biological and adoptive parents. If the adoptive parents and the adopted child’s behaviors and traits correlate, in other words, are more similar than are the behaviors and traits compared to their biological parents, it can be assumed that nurture is responsible for the traits and behaviors, whereas if the adoptive child’s behaviors are more similar to their biological parents, nature is considered responsible for the person they have become. 

What do adoption studies tell us?

According to Kenneth Kendler et al., who compared home-reared and adopted-away co-siblings in Sweden, adopted children provide one of the strongest tests of the environmental malleability of cognitive ability. The research showed the IQ scores of adopted children had higher correlations with the occupational status of their biological parents than their adoptive parents. This suggested that IQ is more a product of nature that nurture. A 2015 meta-analysis of some 14 million twin pairs by Tinca Polderman et al. also discovered that genetics (nature) explained 57% of the variability in cognitive functions. 

Are adopted kids more like their biological parents?

After numerous studies of personality, research shows that the personalities of adopted children are more like their biological parents

How can adoption studies be used to help answer the nature vs nurture debate?

Because the interaction between nature and nurture is complex, researchers have compared the behaviors of adopted people to their adoptive and biological parents in order to tease out if genes or environments determine who people are. By correlating traits like intelligence, addiction, and personality to adoptive and biological parents, researchers are able to establish to what extent people are a product of nature or nurture. 

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