Why is the heritability of mental disorders estimated?

Heritability represents the genetic information transferred from one generation to the next. It is applicable to physical traits, such as height, and to behavioral ones, such as disorders (for example schizophrenia). It is not a new concept, but lately the scientific community has a revitalized interest in it, aiming to estimate it. Especially when it comes to mental disorders, researchers struggle to comprehend the concept and its implications, and find new and precise ways to estimate and measure it.

Specifically, mental disorders are complex traits, meaning that they are caused by many factors. On the one hand these factors are environmental, and on the other hand they are due to genetics; mental disorders are partially heritable, they are transmittable from parents to offspring. By trying to come up with estimates of mental disorders, researchers desire to disentangle the two influences- genes and environment- and detect which part of the variance among individuals is due to the one, or due to the other. The genetic influence can be explicitly seen, whereas the environmental aspect can be implicitly detected after specifying the genetic one, since any part of the variance that remains unexplained by genetics should be due to environment (Brookfield, 2012). On top of that, the influence of heritability may change over time in comparison to the environmental effects, which is another reason why researchers wish to study it. The percentage of the variance explained by genetics can decrease- or increase- over time- for example this could occur in adolescence, where teenagers begin to be increasingly more influenced by their environment (so for example one could have anxiety symptomatology due to peer rejection) (Uher, 2009). Moreover, complex traits are shaped by many genetic factors as well: aside from the environmental influence, the genetic aspect is formed by many small factors. Therefore, plenty of genes are involved in the formation of a mental disorder- and by estimating heritability, scientists try to identify these specific genes, and the precise amount of influence they have on the disorder (Boehnke et al, 2009). In fact, researches have revealed that the genetic component of mental disorders is quite strong (Uher, 2009). Despite that, for each trait studies have identified such a wide variety of genes underlying it- Singly Nucleotide Polymorphisms through Genome Wide Association Studies to be more specific, that heritability as a whole has been even characterized as to have gone missing (Boehnke et al, 2009). In this manner, researchers also struggle to find the most efficient angle to look at the issue, and the most precise methods to do so; therefore heritability can be studied to find applicable ways of how to look at a trait or characteristic.

Here though lies a paradox: why are mental disorders heritable, and how is it possible that they have not been eliminated from the genetic pool? Mental disorders appear to defy the laws of natural selection that are true for so many other traits. Also, ‘mental illness usually has an onset early in the reproductive age and is associated with substantial reproductive disadvantages’, fact which strengthens the question of how are they still heritable. An effort to resolve this paradox is yet another reason to estimate heritability (Uher, 2009, 1073).

Through gaining a better understanding of the causation, researchers are interested in identifying risk factors that may lead to mental disorders, as well as populations at risk. Overall, the ultimate goal of estimating heritability is to be able to develop new treatments and early interventions, as well as preventative measures for mental disorders. Aside from the theoretical aspect of research, the practical goal is to be able to help as much and as efficiently as possible the individuals in need. This can only be achieved after having gained a complete understanding of the causation of mental disorders. Heritability estimates can help in the prevention of disorders since they would indicate factors that may put individuals at risk (Boehnke et al, 2009).

Researchers study heritability in order to gain a better understanding of the causation of mental disorders, and the link of genetics and environment, as well as the precise genes involved, develop improved means to investigate them, and be able to intervene appropriately to disorders and their symptomatology.

References

Boehnke, Cardon, L., R., M., Cho, J., H., Clark, A., G., Chakravarti, A., Collins, F., S., Cox, N., J., Eichler, E., E., Gibson, G., Goldstein, D., B., Gurrmacher, A., E., Haines, J., L., Hindorff, L., A., Hunter, D., J., Kong, A., Kruglyak, L., Mackay, T., F., C., Manolio, T., A., Mardis, E., McCarroll, S., A., McCarthy, M., I., Ramos, E., M., Rotimi, C., N., Slatkin, M., Valle, D., Visscher, P., M., Whittemore, A., S. (2009). Finding the missing heritability of complex diseases. Nature, 461 (8), 747-753

Brookfield, J., F., Y. (2012). Heritability. Current Biology, 22 (7), 217-219

Uher, R., (2009). The role of genetic variation in the causation of mental illness: an evolution- informed framework. Molecular Psychiatry, 14, 1072-1082