The researchers based their study on the reasoning that cause-and-effect relations cannot run backwards in time. If one factor causes another, using the cause to predict future values of the effect should yield more successful predictions than using it to predict past values of the effect. In this way, they could calculate strengths of associations between past and future values of body mass index and personality and infer which is likely to be the cause and which to be the consequence.

Neither personality traits nor individual answers to personality assessment statements could predict changes in body mass index. However, body mass index could predict future changes in the personality traits of conscientiousness and agreeableness. Individuals with lower body mass index values (i.e., lower body weight for their height) tended to a tiny bit more conscientious and agreeable on average compared to those with higher body mass index values.

Conscientiousness is a personality trait characterized by the tendency to be organized, responsible, goal-oriented, and reliable, often associated with being diligent, punctual, and detail-oriented in one’s actions and work.

Agreeableness, on the other hand, is a personality trait reflecting a person’s proclivity to be cooperative, kind, empathetic, and generally pleasant to be around. Agreeable individuals emphasize harmonious social interactions and a willingness to compromise in relationships.

The associations were consistent with causal influences of body mass index on these personality traits, suggesting that either body weight or health status more generally may influence them. Body mass index may thus be among the currently poorly understood correlates of personality development. In contrast, although personality traits were collectively able to predict future body mass index, no clear support was found for causal influences of personality traits on body mass index.