The impact that parents have on their children’s personalities continues to be a topic of debate. Children may, on average, develop a disposition similar to their parents, yet this simple association does not provide information as to why this is so. Parents endow genetic composition to their children and construct the home environment in which their children will grow up. Likewise, sibling’s personalities tend to be more similar than any two people selected at random, but the extent to which the similarity is due to genetics, being raised in the same home and neighborhood, or a combination of the two, is not clear.
In our study (Dunkel, Nedelec, & Van der Linden, 2018), we used a genetically-sensitive research design to further disentangle the environmental and genetic influences on personality. Monozygotic twins, also called identical twins, basically share the same genetic composition, thus any difference between two identical twins must be due to the environment. However, there are two types of environments: factors that twins share which tend to make them more similar (termed shared environment) and factors that twins do not share which tend to make them less similar (termed nonshared environment).
Given that MZ twins share both their genetic and shared environmental factors, any difference in personality (or any other observable trait) must be due to nonshared environmental factors that are unique to each individual twin. A potential example of a nonshared environmental factor that could impact differences in personality is differential parental treatment.
To isolate a potential influence of parental treatment on personality, the difference in personality between 349 adult twin pairs was correlated with the difference in the maternal and paternal affection received. The participants were asked to report on the level of maternal and paternal affection they received when growing up and were also administered a personality questionnaire. The personality questionnaire included scales of the so-called Big Five personality traits of Openness, Conscientiousness, Extraversion, Agreeableness, and Neuroticism. Additionally, the Big Five have consistently been found to cohere to form a General Factor of Personality. Those high on the General Factor of Personality score high on Openness, Conscientiousness, Extraversion, and Agreeableness and low on Neuroticism and are assumed to have a relatively high social effectivity.
It was found that the twin who reported having received more affection was more likely to also possess a higher General Factor of Personality relative to their co-twin who reported receiving less affection. As the General Factor of Personality is similar to Emotional Intelligence and thought to reflect interpersonal skills or social-effectiveness, one implication from the results is that the twin who received greater affection was more likely to develop a more socially-effective personality. One could imagine, for example, that a more affectionate parent would spend time and resources training their children, teaching them how to behave, and comforting them when things go wrong.
Yet many questions remain. An alternative explanation is that the twin with a higher General Factor of Personality was not treated preferentially by their parents in any meaningful way, but simply has a more positive set of recollections. Or that the more personable twin elicited more affection from their parents. Additionally, if the assumption is that the arrow of influence points from parental treatment to personality it is not clear whether the absolute treatment caused the effect or the differential treatment caused the effect. That is, does receiving low parental affection itself have an inverse impact on personality or is the inverse effect on personality the result of being shown less affection relative to the affection shown to that of a sibling; a case of “mom likes you best.” Subsequent research lends support to the possibility that differential treatment by parents is especially important.
These findings are described in the article entitled Using monozygotic twin differences to examine the relationship between parental affection and personality: life history account, recently published in the journal Evolution and Human Behavior. This work was conducted by Curtis S. Dunkel from Western Illinois University, Joseph L. Nedelec from the University of Cincinnati, and Dimitri van der Linden from Erasmus University Rotterdam.