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  • Ajay Macherla

Understanding the Genetics of Personality Traits






Abstract


The aim of this paper is to investigate the heritability of personality traits in humans. We will attempt to answer the following research question:


To what extent are personality traits heritable in humans based on current literature?


The literature review will employ a retrospective analysis and evaluation of three studies pertaining to the research investigation: Jang et al. (1996), Power and Pleuss (2015), and Bratko and Vukasovic (2015). Specifically, we will focus on the five personality traits in the FFM. The review found differences between Jang et al. (1996) and Power and Pleuss (2015) about estimating heritability of personality, and the third study, Bratko and Vukasovic (2015), investigated a possible explanation behind differences in the results of these studies.


It was determined that, from the third study, random participants and families are a more accurate and generalizable study population to use in heritability of personality studies because it does not account for genetic influences in the genome. Twin studies do account for these differences, which is why it cannot be generalizable to a large population, which does have these genetic influences. These results are why Jang et al. (1996) found higher correlations in data; they were concerned with twin pairs. Power and Pleuss (2015) had 5,011 European adults and, although they found significant correlations in heritability, it was only present in two personality traits in the FFM and they were more moderate than the estimations in Jang et al. (1996).


At the end of the investigation, it was shown that Power and Pleuss (2015) is a more accurate and generalizable study that can answer the research question. It was found that certain personality traits have a low to moderate heritability in random humans based on current literature.


However, researchers and psychologists should expound upon the conclusion and either refute or support the findings in this study. They should also make an effort to distinguish between twin and random participant studies in order to account for genetic influences. In doing so, they can ensure that their results are not under or overestimated.



 

Introduction


A primary differentiating factor among the many humans in the world is personality. Whether it may be different thought processes, emotional responses, and behavioral tendencies of an individual, personality traits are unique from individual to individual. But the etiology, specifically its genetics, of personality has yet to be delineated and discovered.


It has long been known that genes are inherited from parent to offspring (Sanchez-Roige et al., 2017). However, there is little literature of the heritability of personality traits. Although correlations have been established in the past, it is difficult to determine whether this relationship can be attributed to genetics or simply the environment, which is assumed to be similar for parents and offspring (Krueger et al., 2008; Sanchez Roige et al., 2017).


The research question for this investigation is as follows:


To what extent are personality traits heritable in humans based on current literature?


Inspiration for this investigation actually came from a realization of the author that certain personality traits aligned with those of his father, and other traits aligned with those of his mother. This correlation made the author wonder whether this dynamic is biologically and genetically explained for. As a result, this research investigation attempts to explore the heritability of personality traits from parent to offspring.


Personality traits investigated in this paper will be categorized in the Five Factor Model (FFM) of personality structure (Widiger & Crego, 2019):

  • Neuroticism

  • Extraversion

  • Openness

  • Agreeableness

  • Conscientiousness

 

Methods

This literature review will employ a retrospective analysis and synthesis of three studies on the heritability of personality traits. To answer the research question, the following procedure will be implemented.


First, a discussion of relevant studies will be presented in a five-step analysis framework, which will help facilitate and streamline an analysis of the results. First, the aim of the study will be presented. Then, the methods in which the experiment was conducted, along with demographic information of the participant sample. After that, the quantitative and qualitative results will be provided, along with a discussion of how the results relate to the research goal. Finally, a conclusion of the study will be presented and its applicability to this review’s research goal.


This literature review will also provide a critical evaluation of the studies presented by including both strengths and limitations. This assessment will allow for a greater appreciation of each study’s relevance to the research goal.


Finally, after synthesizing the information from all the studies, the author will attempt to answer the research question. A claim, followed by evidence and reasoning, will be set forth to do so.



 

Results

Jang et al. (1996):

Aim:

  • To investigate the heritability of the big five personality traits in FFM through identical and fraternal twins

Method:

  • Participants: 123 pairs of identical twins, 127 pairs of fraternal twins

  • Procedure:

  • Twins completed the NEO Personality Inventory (NEO-PI-R), containing 240 items for scoring in each of the personality dimension

  • Each dimension was operationalized as having six scales. Details of each specific scale of each trait will not be provided; access the original study in the according source.

  • Estimated heritability using square root or natural logarithmic transformations, used the computer program PRELIS 2 to compute the estimations

Results:

  • Found the following heritability estimates for each trait of FFM (P<.05)

  • Neuroticism: 41%

  • Extraversion: 53%

  • Openness: 61%

  • Agreeableness: 41%

  • Conscientiousness: 44%

Conclusions:

  • Results showed substantial heritability in each of the five dimensions of FFM

  • Environmental and genetic influences accounted for the variance between the scales

  • May be a combination of genetic and environmental factors that contribute to the heritability of the traits

Strengths:

  • Large sample size, no genetic differences between the twins, high control of data, high reliability, no real ethical considerations


Limitations:

  • Environmental factors may have played a confounding role in the results, low generalizability to all populations

______________________________________________________________________________

Power and Pluess (2015);

Aim:

  • To estimate the heritability of the personality dimensions in FFM through genomic-relatedness-matrix residual maximum likelihood (GREML) analysis

Method:

  • Participants: sample of 5,011 European adults

  • Procedure:

  • Identified sample of participants in the study

  • Gathered genetic data from “the Illumina Human Hap 550 K v1.1 platform” (Power & Pluess, 2015)

  • Participants were given a FFM questionnaire using 50 measures form the International Personality Item Pool (IPIP)

  • Noted that IPIP correlates strongly with NEO-PI-R

  • Performed GREML analysis utilizing genome-wide complex trait analysis software (Yang et al., 2011)

Results:





NOTE: All figures obtained from original study source

Conclusions:

  • Significant heritability for neuroticism and openness personality traits in FFM of 15% and 21% (P=0.04, P<0.01)

  • Low correlation for the other three traits in FFM

Strengths:

  • No ethical considerations, confident correlation established, large sample size, high reliability, strong analysis procedures using GREML

Limitations:

  • Low generalizability to all populations, low cross-cultural validity, no significant correlation for agreeableness, conscientiousness, and extraversion

______________________________________________________________________________

Bratko and Vukasovic (2015):

Aim:

  • To analyze and synthesize findings on the heritability of personality through a meta-analysis, test whether study design, personality model, and gender

Method:

  • Participants: a pool of 134 studies with 62 effect sizes were identified

  • Procedure:

  • Studies included over 100,000 participants in total

  • Gender and age for controlled for

  • Data analyzed utilizing random-effects model and “software program R package metafor” (Vukasovic & Bratko, 2015).

Results:

  • Average effect size was 0.40, meaning that 40% of differences in personality can be attributed to genetics

  • Gender did not play a significant role in heritability of personality

  • Twin studies showed higher estimates than those without twins in participant sample

  • Average of 0.47 effect size compared to 0.22 of family studies

Conclusions:

  • Factors of the study design in studying the heritability of personality does impact the results of the findings

  • Complicates the investigation of heritability of personality traits

Strengths:

  • Large sample size, accounted for gender and other factors of study design in the topic, high reliability, no real ethical considerations

Limitations:

  • Shows that twin studies and other research on personality heritability may be slightly skewed, complicates investigation


 

Discussions

Synthesizing the information from the three studies, we can discuss the findings gathered and analyze its relevance to the research question.


Jang et al. (1996) saw a moderate to high correlation between the big five personality traits in FFM and heritability (P<0.05). The low p-value showcases that the results are significant and the researchers can be confident about their validity. It seems that the heritability of the traits are between 40-60%, but after looking at the information in the other two studies, the situation is complicated. Power and Pleuss (2015) attempted to estimate the heritability of the traits in FFM and only found a significant correlation in two traits: neuroticism and openness.


Let us consider the reasons as to why these differences in results may occur. One, the research design. As Bratko and Vukasovic (2015) points out, the study design does have an effect on the effect size and thus the determined heritability estimate. It was shown that twin studies typically have higher estimates than studies with families. This association was evident in the participant sample of Jang et al. (1996), which consisted of more than 200 pairs of twins, and Power and Pleuss (2015), which had a sample of 5,011 European adults who were not twins. The results in Bratko and Vukasovic (2015) thus explain why there is a difference between results in these two studies. For this reason, it is unclear as to what is the best study design and conditions to effectively carry out an experiment on heritability of personality traits that generates accurate results.


One aspect that does seem clear is that gender does not play a significant role in skewing the results. This possibility means that future researchers are not obligated to control for gender, as past empirical evidence has showcased that it does not significantly impact heritability. However, as a best practice, it is important that researchers do so.


Jang et al. (1996) and Power and Pleuss (2015) are examples of the different results that can be gained if researchers utilize twins in their study, compared to utilizing random participants or families.


This result may be attributed to the fact that using twins in a study concerning heritability controls for the effects of other genes in the genome on the heritability. For this reason, higher correlations can be established because there are less limiting genetic factors that may skew results. In contrast, families or random participants have different genes, which may pose as a confounding variable in gathering and analyzing the results of these kinds of studies. However, twin studies are not generalizable to the entire population, as there are significantly less pairs of twins in the world.


From a generalizability standpoint, it can be reasoned that utilizing random participants and families may be a better way of gaining more accurate results, as the effect of other genes in the genome is considered in the final results. The findings gained from these studies may be more conducive to the entire population compared to twin studies. Of course, if the researchers aim to study the heritability of personality traits in twin populations, then twin studies are more appropriate.


From these results, it can be argued that random participants and families are a more effective and accurate way of gathering data about the heritability of personality traits in the FFM. Thus, we can consider the results from Power and Pleuss (2015). Since the study argued that only neuroticism and openness had significant heritability estimates of 15% and 21%, with p-values less than 0.05, it can be seen that, in a general population, there is a low to moderate heritability of personality traits.


 


Conclusions

After synthesizing the results from the three studies, we can come to several conclusions. Again, we will attempt to answer the following research question:


To what extent are personality traits heritable in humans based on current literature?


First, considering Brakto and Vukasovic (2015), random participants and families seem to be a more accurate and generalizable sample for larger populations. If we aim to effectively answer the research question from the standpoint of the general public, twin studies may not be the most accurate method of gathering results as it controls for mostly genetic influences in the genome, an effect that is not present in the majority of the population.


For this reason, considering Power and Pleuss (2015), neuroticism and openness are two personality traits of FFM that have a significant correlation of heritability, with 15% and 21%. Since the participant sample was adults in Europe, with no genetic relationships, it can be inferred that these results are more generalizable to larger populations. Thus, the following answer to the research question can be posed:


Certain personality traits, such as neuroticism and openness, have a low to moderate heritability in humans based on current literature.


Despite this conclusion, it is important that researchers and psychologists continue to expound upon current literature on the heritability of personality traits. As for next steps, it is important that researchers look at personality traits outside of the FFM traits and distinguish between the effect size of twin studies versus family studies, as Brakto and Vukasovic (2015) suggests.


From this paper, researchers have a better idea of the study design of an effective study on the heritability of personality. Furthermore, they have a reference point––the conclusion drawn in this literature review––that will allow them to have streamlined research efforts to either refute or elaborate on this conclusion.



 

References

Jang, K. L., Livesley, W. J., & Vemon, P. A. (1996). Heritability of the Big Five Personality Dimensions and Their Facets: A Twin Study. Journal of Personality, 64(3), 577-592. https://doi.org/10.1111/j.1467-6494.1996.tb00522.x


Krueger, R. F., South, S., Johnson, W., & Iacono, W. (2008). The Heritability of Personality Is Not Always 50%: Gene-Environment Interactions and Correlations Between Personality and Parenting. Journal of Personality, 76(6), 1485-1522. https://doi.org/10.1111/j.1467-6494.2008.00529.x


Power, R. A., & Pluess, M. (2015). Heritability estimates of the Big Five personality traits based on common genetic variants. Translational Psychiatry, 5(7), e604. https://doi.org/10.1038/tp.2015.96


Sanchez-Roige, S., Gray, J. C., MacKillop, J., Chen, C.-H., & Palmer, A. A. (2017). The genetics of human personality. Genes, Brain and Behavior, 17(3), e12439. https://doi.org/10.1111/gbb.12439


Vukasović, T., & Bratko, D. (2015). Heritability of personality: A meta-analysis of behavior genetic studies. Psychological Bulletin, 141(4), 769-785. https://doi.org/10.1037/bul0000017


Widiger, T. A., & Crego, C. (2019). The Five Factor Model of personality structure: An update. World Psychiatry, 18(3), 271-272. https://doi.org/10.1002/wps.20658


Yang, J., Lee, S. H., Goddard, M. E., & Visscher, P. M. (2011). GCTA: A Tool for Genome-wide Complex Trait Analysis. The American Journal of Human Genetics, 88(1), 76-82. https://doi.org/10.1016/j.ajhg.2010.11.011



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