Before completing the module below, please read the section of the textbook called Twin studies, family studies, and adoption studies. When reading this section, please take notes on the following important concepts:

• Adoption studies
• Concordance rate
• Equal environment fallacy
• Family studies (pedigree studies)
• Genetic vulnerability
• Monozygotic vs dizygotic twins
• Prospective research
• Selective placement

• Family studies are often based on anecdotal data - that is, asking family members about the behaviour of other members of the family.  In some cases, behaviour may be documented - for example, a diagnosis of bipolar disorder.  However, often this is not the case, so reports of one's sexuality, memory capacity or intelligence may be based on family stories, rather than on actual data. In some cases, such as Alzheimer's disease, the period between generations may be too long to allow for meaningful data.
• Twin studies allow researchers to look at the effect of genetic inheritance while keeping environmental variables relatively constant.
• Twins do not represent the general population, so the research may not be generalizable. For example, there may be genetic factors that lead to higher rates of twins in some women.  In addition, twins are raised with someone that looks just like them and shares their experiences.
• Most of the twin registry databases from which samples are drawn are from the developed world. The most commonly used are in Sweden, Denmark, Norway, Finland, Australia, the USA, and the United Kingdom. The Sri Lankan Twin registry, established in 1996, is the only existing population-based Twin Registry in a low-income country.
• Adoption studies allow us to compare a child to their biological and adoptive parents.  If their behaviour is more similar to their biological than their adoptive parents, this indicates a possible genetic root of the behaviour.
• Children that know that they are adopted are not representative of the general population.
• A large amount of adoption research focuses on the behaviour of the birth mother, rather than on the father.

Wender et al (1986) wanted to see the role of genetic and environmental factors in the origin of depression. To do this, they studied biological and adoptive relatives of adult adoptees with depression and in matched adoptees who did not suffer from depression. Psychiatric evaluations of the relatives were made on the basis of independent blind diagnoses based on hospital records. Analysis of the data showed that depression was eight times as prevalent in the biological parents of the adoptees with a diagnosis of depression over the adoptive relatives; suicide was 15 times more common in the biological parents.

Kendler el al (2018) sought to determine the role of environment in the origin of major depression. They used a sample made up of 2,596 Swedish siblings who were high-risk for major depression -  where one sibling was raised with the biological family and one with an adoptive family.  National medical registries were used to assess a history of major depression. They defined high-risk as having at least one biological parent with major depression.

The researchers reported that major depression risk was reduced by 23% (95% CI, 7-36) when a child from a high-risk family was adopted. This difference disappeared when a step-sibling or adoptive parent had major depression or the adoptive home was disrupted by divorce or parental death.