Personality Traits: Genetic or Environmental?

I feel the most secure when I feel everything is under my control: my grades, my relationships, and mostly, who I am. However, what truly shapes people's characteristics? Most people would say their upbringing or major life events that have changed the course of their life. Hopwood et al. aimed to determine how genetic and environmental influences on personality occur during transition to adulthood.

Introduction

Previous similar studies on human traits have debated two topics: the intrinsic maturation hypothesis and the life-course hypothesis. The latter sounds much like one would expect the term to be- the hypothesis that growth comes mainly from outside circumstances. The former describes the idea that development of someone's characteristics and behavior come internally.

In order to secure the data for the study, Hopwood et al. gathered a group of same-sex twins (male and female) and gave them the Multidimensional Personality Questionnaire (MPQ). The MPQ tests for 4 main personality traits and unlike the famous Meyers-Brigg Personality Test (I'm a INFJ, apparently the rarest!), individuals aren't categorized.

Background

Researchers chose to gather data of twins from three age groups: 17 (which they called adolescence), 24 (emerging adulthood), and 29 (young adulthood). This decision stems from the belief that most transition between these ages involves significant personal change which can come in outside or inner factors.

Although many similar studies have been carried out (such as the experiment done by Blonigren et al.), Hopwood et al. aimed to create a study that acts as an extension of previous works to fill in gaps of knowledge and have chose three waves (three age groups) to do so.

Created in 1982, the MPQ looks at the ability to sense someone's emotions without communication, motivation, behaviors while interacting with people, conformity, etc. However, the 4 broad traits are positive emotional temperament, negative emotional temperament, constraint, and absorption. The study will focus on only three.

Positive emotional temperament (PEM) is a personality trait when an individual naturally experiences a high level of positive emotion, while negative emotional temperament (NEM) is a trait characterized by frequent negative emotions. Constraint (CON) measures how external or internal factors affect behavior, decision-making, and thinking.

Additionally, the experiment compares both absolute change, which is average change in a group. Think something similar to mean (if you still remember algebra or statistics!). On the other hand, differential stability describes consistency in individuals.

Analysis

Hopwood et al. collected these results:

• Personality traits are as heritable in late adolescence as in young adulthood.

• Stability in environment increased as age increased (after emerging adulthood).

• There were the most changes in PEM and CON (mostly individual change).

As age of the study participant increased, negative emotionality decreased drastically while constraint increased drastically as well. (Other measured personality traits had little to no change)

The figure above highlights the finding that negative traits subsided greatly while control of behavior in response to events increased greatly as well. Thus, it was found that individuals go through a great change in personality late adolescence and through emerging adulthood, and overall became much more mature.

As shown in the pie charts to the left, causes of variance of traits between the twins (at age 17, late adolescence) are shown. Non-shared environments were most often the cause of variance in traits between twins, with genetics in second and shared environment in third. It is surprising that the effect of shared environments is so low, assuming that twins were raised and grew up in the same house.

 Typically, across the three ages, shared environments did not affect NEM much. However, although it may seem as adolescents have more similar environments (especially because they attend school, it was found that more young adulthoods had traits affected by shared environments (most likely the choice to similar workplaces and marriage/raising families). Additionally, it was surprising to see that genetics played a large role in variance.

Results for CON (control of behavior) between twins at the age of 17 and 29 are very similar.

Results

Overall, Hopwood et al. found results correlated with the intrinsic maturation hypothesis. Additionally, it was concluded that important developmental periods can be characterized by different levels of stability, with more instability from late teens to mid 20s and the most stability after the age of around 24.

CON was found to be involved in a person's ability to control emotional response to events and was linked to the forebrain (the brain right below the top of your head). Additionally, NEM was linked to a structure in the brain that stabilizes as people age. Therefore, it was confirmed that because the structure stabilized as people age, adolescents are more extremely responsive to outside events.

Although genetics contribute to stability and change, most of trait changes can be accounted for by environmental factors. As people grew older, they chose to create stability promoting roles (corporate jobs, etc.) and relationships (families, marriage, etc.). So, these new environments that adults aged 24+ put themselves into promoted less change, leading to less chance for a change in personality.

However, this study does have its faults. The MCQ was taken by individuals themselves so the study relied on self-reported personality traits (I won't lie, I've made myself look better during personality traits too even when only I see them...). And environments may also have been exaggerated or over-estimated.

In conclusion, trait changes are more common in adolescence and early adulthood and stable in the second half of early adulthood. As people age, it was found that there was a decrease in negative emotions and minimal change due to genetic and environmental factors.