Let's Celebrate the Big Things!

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Week 2 - How have twin studies contributed to our understanding of aetiology of mental health?

Plomin et al. (2016) describe the Top 10 replicated findings from behavioural genetics - a cause for celebration in pychology! Big and counterintuitive findings start to change the psychological perspective about the origins of individual differences in behaviour. This blog focuses on findings related to twin studies, and their contributions to our understanding of aetiology of mental health.

Pairs of identical and fraternal twins are compared via cognitive abilities/disabilities, psychopathology, personality, and more. The first finding states that all psychological traits show significant and substantial genetic influence. This finding is so widespread that it proves great difficulty to find any trait for which genetic influence is not significantly different from zero in more than one study. The second finding states that no traits are 100% heritable. This is a crucial finding because it provides the strongest evidence for the importance of environmental influence after controlling for genetic influences.

Consider the relationships between any two traits, such as creativity and mental health, or empathy and moral behaviour. In the rare situations that genetically informed designs are used to find the correlations between traits, they point to the far-reaching implications of the finding that phenotypic correlations between psychological traits show significant and substantial genetic mediation. Genetic factors consistently account for more than half of phenotypic correlations (Davis, Haworth, & Plomin, 2009). More than 100 twin studies address the key question of comorbidity in psychopathology. For instance, a review of 23 twin studies and 12 family studies confirmed that the same genes affect both anxiety and depression. This means that, from a genetic perspective, they are essentially the same disorder. Twin studies consistently show substantial genetic overlap between common disorders in children and adults.

Large genetic effect sizes are the most important reason for the reproducibility of results. It is amazing that inherited DNA differences can work through the complexities of pathways to account for so much variance of complex psychological traits.

This research sends the overall important, yet simple, message that both nature and nurture contribute substantially to individual differences in psychological traits.

References

Plomin, R., DeFries, J. C., Knopik, V. S., & Neiderhiser, J. M. (2016). Top 10 replicated findings from behavioral genetics. Perspectives on psychological science, 11(1), 3-23.

Davis, O. S., Haworth, C. M., & Plomin, R. (2009). Dramatic increase in heritability of cognitive development from early to middle childhood: An 8-year longitudinal study of 8,700 pairs of twins. Psychological Science, 20(10), 1301-1308.

My second psychology focused blog post, I hope these perform well. I have nine in total to share, that means nine weeks total of consistent blog posts from me, hehe!

Signing off,

burnt toast, sweet tea;