Genetic research in humans is to a large extent based on correlational studies. This means that a researcher establishes that there is a relationship between variables, but the researcher does not manipulate an independent variable as in an experiment. Therefore, no cause and effect can be determined. Since the completion of the Human Genome Project, we have seen a change in how genetic research is carried out.

Twin studies, family studies, and adoption studies

One of the ways to study the possible correlation between genetic inheritance and behaviour is through twin research. Researchers study twins because they share common genetic material. Prior to the Human Genome Project, it was the primary way that psychologists investigated the role of genetics in human behaviour.

Psychologists use these different degrees of a genetic relationship as a basis for their hypotheses about the contribution of genetic and environmental factors to behaviours such as psychiatric disorders or addictive behaviour. It should be the case that the higher the genetic relationship, the more similar individuals will be if the particular characteristic being investigated is inherited. In twin research, the correlation found is called the concordance rate. Twin research is based on a systematic analysis of the similarity between MZ and DZ twins and based on the assumption that any heritable trait will be more concordant in identical twins than in non-identical twins, and concordance rates will be even lower in siblings.  When carrying out twin research, if the concordance rate for MZ twins is significantly higher than for DZ twins or siblings, it is likely that there is a genetic component to the behaviour. If the concordance rate is high for both MZ and DZ twins it may be assumed that environmental factors play a large role in the observed behaviour.

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There is no genetic research that says that obesity is 100% the result of genotype - although there are some types of obesity that may have a genetic link.

The example from the press report is highly problematic.  It appears that MZ twins are more likely to both be obese than DZ twins - and this presents the possibility that obesity has a genetic root.  However, students should be able to talk about the following:

• When twins live together, it is difficult to isolate environmental factors. They would both be exposed to the same diet and eating behaviors within the family.  This does not, however, explain the difference between the twins.
• As the concordance rates are not 100% for MZ twins, one has to wonder whether obesity is not genetic - or whether they have the genes for obesity, but that they have not had an appropriate stressor to lead to gene expression.
• The word "prove" is always problematic in psychology.  Scientists try to "disprove" a hypothesis (the null), but do not prove it.
• In addition, this could be a self-fulfilling prophecy.  Many people who struggle with their weight might find such news positive since it means that they do not have control over their weight! (Phew) There is a problem that reporting such news may actually deter people from healthy exercise and eating habits.

It makes sense for psychologists to first carry out twin studies to determine the concordance rate for a behaviour between twins before carrying out more sophisticated genetic research, which is much more expensive. However, there are limitations of twin studies. First, twins are very rarely “raised apart,” so they tend to experience a very similar environment while growing up.  Therefore, it is difficult to isolate environmental influence as a variable.  That being said, we have to be careful not to overestimate the similarity of the environment for both twins.  Assuming that twins grow up in an equal environment is often called the equal environment fallacy. Some research suggests that parents, teachers, peers, and others may treat identical twins more similarly than fraternal twins.  Finally, twins are not highly representative of the general population, so it is difficult to generalize the findings.

Another way that behavioural genetics is studied is through family studies (also called pedigree studies). Unlike twin research, this is a more representative sample of the general population. A child inherits half its genes from the mother and half from the father. It follows that ordinary brothers and sisters will share 50 percent of their genes with each other; grandparents will share 25 percent of their genes with their grandchildren, and first cousins will have 12.5 percent of their genes in common. In family studies, these different degrees of genetic relatedness are compared with respect to specific traits or behaviour. The notion is that concordance rates will increase if heritability is high and vice versa. For example, if the heritability of IQ Intelligence quotient is high, there should be a stronger correlation in IQ between children and their mothers than the correlation in IQ between second cousins, and very little, if any, between strangers.

When a behaviour is suspected of being genetic within a family, psychologists use prospective studies, that is, the sample is selected and observed before certain behaviours are observed. The researchers watch for outcomes, such as the development of a disorder.  For example, individuals who are considered “genetically vulnerable” for schizophrenia can be followed over many years to see if they actually develop the disorder. However, there is an ethical concern that such research may cause undue stress to those who are labeled as vulnerable.

A final method used in traditional genetic research is adoption studies. In principle, these allow the most direct comparison of genetic and environmental influences on behaviour. Adopted children generally share none of their genes with their adoptive parents, but they do share 50 percent of their genes with their biological parents. It would be reasonable to suppose, therefore, that if the heritability of a behaviour is high and the environment has little part to play, then the behaviour of adopted children should correlate more strongly with the behaviour of their biological parents than their adoptive parents. If, on the other hand, the environment has the strongest role to play, the reverse pattern should be found.

Adoption studies are often criticized as these children are not representative of the general population. In addition, adoption agencies tend to use selective placement when finding homes for children, trying to place children with families who are similar in as many ways as possible to the natural parents. Consequently, the effects of genetic inheritance may be difficult to separate from the influences of the environment.

Overall, these approaches to the study of the relative influence of genetic makeup and the environment allow researchers to determine if there is a potential genetic origin of behaviour. In spite of the weaknesses outlined here, it is clear that there is a correlation between several behaviours and genetic inheritance.

Genetic research after the Human Genome Project

After the completion of the Human Genome Project, one of the key ways in which researchers study the heritability of a trait is through genetic mapping – also known as linkage analysis. Genetic mapping indicates which chromosome contains the gene related to the behaviour, as well as where the gene is located on that chromosome.  To create a genetic map, researchers collect blood samples from members of the families in which a behaviour is common – for example, schizophrenia or aggression. The researchers examine the DNA for polymorphism – the presence of genetic variation. These polymorphisms are referred to as genetic markers.

Association studies look to see if there is a correlation between these genetic markers and a certain behaviour. Although this is much more precise than the general twin study research outlined above, the results are still correlational in nature.  There would have to be a lot of research on these markers in order to see whether the effect is significant.  That is where GWAS come in.

Once a particular gene is suspected of playing a role in human behaviour, researchers today carry out Genome-wide association studies – known as GWAS. These studies compare the DNA of two groups of participants: people with the behaviour and similar people without, who serve as controls. If a genetic marker is more frequent in people with the behaviour, it is said to be "associated" with the disorder. As you can see by its name, GWAS by definition is  a very large study – often using data from hundreds of thousands of samples of both control and “target behaviours.” By using such a large amount of data, the effect of outliers does not lead to false conclusions.

To make sense of the data, a graph called a “Manhattan Plot” is generated. Along the x-axis, you can see the chromosomes and the genes that reside on each chromosome. The y-axis indicates the level of significance for the association of a genetic variation with a behaviour.  The red line is the required level of significance to determine whether a gene variation may exist between a control group and people living with schizophrenia.  By looking at the peaks, we can see that there are several genes that appear to differ between the two groups, indicating that a combination of these genes may lead to schizophrenia. These are known as candidate genes - in other words, that does not mean that researchers now know which gene is the one that leads to schizophrenia, but they have identified genes that now require more research!

Checking for understanding

Epigenetics and depression