The relatedness of PPs—the percentage of genetic similarity between individuals—is referred to as genetic similarities. Researchers frequently investigate the heritability of a certain feature, such as mental disease (depression), using genetic similarities. This is achieved through twin studies, which examine the similar and different behaviours of dizygotic (DZ) and monozygotic (MZ) twins—twins from two different eggs fertilised simultaneously and sharing 50% of their DNA like regular siblings. PPS uses genetic similarity to determine how heritable a trait is: if more genetically similar individuals—like MZ twins or siblings, share the same traits and more frequently than less genetically similar individuals—like DZ twins or cousins. Hence, it can be assumed that genetics plays a key role in determining those traits. This presents a concordance rate of behaviours, or the likelihood that a trait will be shared by both PPs. But because most of these studies are done on families, it can be challenging to determine whether higher concordance rates are caused by environmental or genetic similarities. As a result, it can be challenging to determine precise quantitative measures of a trait's heritability.
Depression's heritability would be one instance of this. Scientists have been trying to determine the precise role genes play in raising or lowering depression. This is because, in accordance with the Diathesis Stress Model, genetic and environmental factors both contribute to depression, making it difficult to determine whether two individuals are depressed due to shared stressful situations or genes.
Study: Kendler et al
As both DZ and MZ twins typically share the same settings but have varying degrees of relatedness, Kendler et al. used twin research to evaluate this. Researchers may then conclude that depression has a significant degree of heredity if the MZ twins have higher concordance rates. They utilised identical twins who were born 32 years apart to do this, and they gave them a condensed version of the Eysenck Personality Inventory. Subsequently, the twins' depression was assessed through phone interviews using the Composite International Diagnostic Interviews (CIDI).
In addition, PPs taking antidepressants had the option to opt out of the "are you sad" question during this interview by indicating that they had a history of depression and were now feeling better because of taking medication. The twins were also questioned about how long they had lived together and how frequently they had met. The twins also self-reported their zygosity. According to the findings, the concordance rate for MZ twins was 44% in females and slightly lower in males, whereas the concordance rate for DZ twins was 16% in females and slightly lower in males. Additionally, it was demonstrated that the quantity of occasions twins lived or shared a residence had no bearing on the outcome. This made it possible for researchers to determine that, based on twin model genetic similarities, 28% of sadness (behaviour) was heritable. The study found that while genes may contribute to a person's susceptibility to depression, environmental factors played a part in this behaviour, as evidenced by the MZ twin concordance rate being less than 100%.
Numerous issues with this study cast doubt on its conclusions, even though it makes a clear comparison between genetic similarities and sadness and even evaluates the possibility that living with twins—sharing the same environment—may have contributed to the results. Statistically speaking, it is difficult to determine the degree of importance with such a huge sample, thus even though the years of living together were deemed negligible, this judgement may be off. Researchers must also consider the fact that, despite their genetic similarity, the MZ twins still share a higher degree of environmental similarity with the DZ twins due to their similar appearance, the fact that they are still typically treated more similarly, and the possibility that they share similar insecurities. All these factors would naturally result in a higher concordance rate for the MZ twins.
In conclusion, distinct methods for comprehending the heritability of traits like sadness are presented by varying degrees and kinds of genetic similarity. However, because it's challenging to gauge how much influence environmental influences have, determining a precise percentage heredity is a challenging task. Because of this, it is frequently difficult to determine a trait's precise heritability.