Twin and adoption studies Flashcards
What is a twin study?
A study where twins are compared on a specific trait to see how similar they are.
If a particular behaviour is entirely genetic then we expect MZ twins to show 100% concordance.
If a particular behaviour is more genetic than environmental then we would expect
MZ twins to show a higher concordance rate than DZ twins.
What are some positives of a twin study?
+ Enables researchers to investigate the influence of genes because it is assumed that both MZ and DZ co-twins share the same environments. The only difference between the two groups of participants is genetic.
Therefore, if a trait is genetic then MZ twins should show higher concordance than DZ twins.
+ Although twins are unusual, information for twin studies is often taken from twin registries (such as the MTFS used in Ludeke et al’s study on facing page). These hold data on thousands of twins and also contain information about many variables. This means that the sample is large and the data is likely to be representative.
What are some negatives of a twin study?
Twin studies may overestimate
genetic influence. MZ twins have more similar environments than same-sex DZ twins - they are treated more similarly, spend more time with each other and tend to share friends. The environmental experience of one MZ twin is to a large extent the same as the environmental experience of the other - much more so than for DZ twins. Therefore, some of the estimated similarity is actually due to a shared environment.
- Twins studies provide a very broad indication that a behaviour/ characteristic has a genetic origin, but they cannot identify the specific genes involved. A twin study is therefore a useful starting point for research, but more recent technologies (see ‘association studies’ on facing page) are required to help us uncover specific genetic influences.
What is an adoption study?
Genetic factors are implicated if children are more similar to their biological parents with whom they share genes (but not environment)
than to their
adoptive parents with whom they share environment (but not genes).
Environmental factors are implicated if the reverse is true.
What are some positives of an adoption study?
+ Adoption studies have the advantage of removing the extraneous variable of environment
- if MZ twins are similar it is not clear whether the similarity is due to the same environment or the same genes. In adoption studies, environment is not shared or biology is not shared.
+ Adoption studies have been useful in showing that twin studies overestimate genetic factors. For example, a study by Eley et al.
(1998) using adoption studies to research depression found that environmental factors are more important, whereas twin studies have detected a greater role for genetics. This may be one area in which the assumption of equal environments is clearly untrue.
What are negatives of an adoption study?
- Children may be adopted to families similar to their biological families (called
‘selective placement’) and therefore environmental influences may be similar.
For example, a child whose biological parents are well-educated is placed with a similar family. This means that apparent similarities with biological relatives may be due to environmental similarities rather than genes. - People who adopt other people’s biological offspring are unusual, so are unlikely to be representative of the population. They tend to be better educated, have lower rates of mental illness and come from higher socioeconomic groups. This means any conclusions we draw from adoption studies about the effects of genes (versus environment) on traits such as intelligence may not be generalisable to the population as a whole.
What is an example of a twin study of Sz?
Gottesman and Shields (1966) - Schizophrenia
Irving Gottesman and James Shields investigated the genetic basis of schizophrenia.
A search through the records of about 45,000 psychiatric patients at the Maudsley and Bethlem hospital in London, found 57 pairs of twins where at least one had a diagnosis of schizophrenia. This was an opportunity sample.
In order to establish concordance they contacted co-twins. Zygosity was determined by looking at, for example, blood type. Therefore, the researchers could identify MZ and DZ twin pairs.
An analysis of this data and data from other studies, suggested that if one MZ twin had schizophrenia, their co-twin was at least 42 times more likely to have schizophrenia than a person from the general population. A DZ twin of the same sex was at least nine times as likely to have schizophrenia if their co-twin had been diagnosed.
What is an example of a twin study of aggression?
Brendgen et al. (2005) - Aggression
This is one of the optional contemporary studies in chapter 3 (see page 114).
Mara Brendgen et al. investigated aggression in MZ and DZ twins to assess the contributions of genetic and environmental factors.
They used a twin study to compare concordance rates for 234 pairs of MZ and DZ twins. Each twin was rated for his or her level of social and physical aggression by peers (other children) and teachers.
Physical aggression was mostly explained by genetic factors, but social aggression was much more influenced by environmental factors. They concluded that heritable characteristics (such as poor self-control) predisposes some children to aggression But the form the aggression takes (social or physical) depends more on the environment.
