How much does size matter? The importance of modest outcomes

Personality has often been dismissed as a predictor of life's outcomes, in favour of predictors such as IQ, generic cognitive ability, income, educational status, and socioeconomic status. This is because it's been well-documented that personality only has a correlation co-efficient of .30, which is categorized as a low-modest correlation.

Whilst on its own, .30 isn't a wildly strong correlation, we shouldn't look at results like these in a vacuum. .30 is actually a pretty average correlation for any two co-variables in psychology, with the strongest correlations often only reaching .50 or .60. But this isn't only the case with psychology, but other human sciences, such as medicine.

Not only is a correlation of .30 actually pretty common, but it is useful. In terms of predicting life outcomes such as divorce rate, mortality rates, and occupational success, it's just as accurate, and at times more so, as cognitive ability, IQ, and socioeconomic status. Neuroticism and hostility are both significantly associated with a diminished life span, for example. So whilst being nasty may not kill you, it's worth knowing that being nice genuinely increases your chances of a long and happy life.

But what about smaller correlations? Well, medicine sees some pretty tiny correlations. Take, for example, that the correlation between taking aspirin and treating heart disease is .02, and the relationship between chemotherapy and treating breast cancer is .03. In many cases, people might dismiss such correlations as non-significant. However, science isn't just about the numbers. It's about people. The findings of these small correlations matter, because chemotherapy can treat breast cancer, and aspirin does reduce heart attacks.

So, correlations less than .40 may be modest, but they really do matter.

References
Roberts, B. W., Kuncel, N. R., Shiner, R., Caspi, A. and Goldberg, L. R. (2007). The power of personality: The Comparative Validity of Personality Traits, Socioeconomic Status, and  Cognitive Ability for Predicting Important Life Outcomes. Perspectives on Psychological Science, 2. DOI: 10.1111/j.1745-6916.2007.00047.x