A study published in Psychological Science evaluated the link between happiness and personality traits in 973 twins. The authors found that happiness was heritable, and that it showed genetic linkage to certain personality traits. Those who were extroverted, open, agreeable and conscientious were more likely to be happy. Moreover, twins who exhibited similar personality traits had similar levels of happiness in a seemingly genetic pattern.

The authors concluded that these results mean happiness could be the result of these specific personality traits, and that depression was brought about by the opposing traits of introversion, disagreeability and neuroticism. While this is a potentially correct conclusion, it seems that the opposite could also be true — depression is an inheritable trait, which leads to certain personality traits that occur in response to this depressed mood.

Many studies have linked certain personality traits to depression. But whether the underlying cause is these traits or depression itself remains to be determined. Alternatively, it may be that both personality traits and depressive tendencies are genetically inherited, but they are inherited together in a predictable pattern, i.e. genes for depressive mood and introversion travel together.

Anyone who has been depressed can vouch for the fact that, when things get bad, you do not want to socialize, you do not feel agreeable or open, and you are not very conscientious about yourself or those around you. So it seems plausible that these traits are as much a product of the depression as they are its underlying cause.

The truth probably lies somewhere in between. Personality traits are not always inherited, and environmental factors clearly also play a role in the development of personality development. Environmental factors are also important in the development of depression — but we still cannot explain why some individuals become depressed but others do not in response to the same stressful event. Underlying genetic susceptibility to depression and other mood disorders clearly exists, and likely interacts with life events and personal habits in a complex fashion to ultimately determine our personality and our mood. But the idea that our personality traits are genetically determined and are the major determinate of mood disorders is still just a theory.

Reference

Weiss, A., Bates, T.C., Luciano, M. (2008). Happiness Is a Personal(ity) Thing: The Genetics of Personality and Well-Being in a Representative Sample. Psychological Science, 19(3), 205-210. DOI: 10.1111/j.1467-9280.2008.02068.x