Are Personality Traits Caused by Genes or Environment?

Different personality traits help make people unique
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Psychologists often talk about personality traits, but what exactly is a trait? How do mental health professionals define this term? It is our personalities that help make us unique individuals, but not everyone agrees on exactly how many different traits exist. Some break personality down into very narrow and specific traits, while others prefer to look at traits much more broadly.

What Are Personality Traits?

Traits are typically defined as the different characteristics that make up an individual's personality.

In the Handbook of Personality: Theory and Research, authors Roberts, Wood, and Caspi define personality traits as "the relatively enduring patterns of thoughts, feelings, and behaviors that distinguish individuals from one another."

Trait theory suggests that our personalities are made up of a number of different broad traits. For example, extroversion (often known as extraversion) is a personality dimension that describes how people interact with the world. Some people are very extroverted (or extraverted) and outgoing, for example, while others are more introverted and reserved.

Until fairly recently, it was believed that personality traits changed very little over the course of a lifetime. Some newer longitudinal studies have revealed that traits are a bit more nuanced than previously believed and that some personality changes can and do occur over time.

Types of Traits

What would you say if someone asked you to list the personality traits that best describe you? You might rattle off a variety of traits such as kind, aggressive, polite, shy, outgoing, or ambitious. If you were to make a list of every personality trait, it would probably include hundreds or even thousands of different terms used to describe different aspects of personality.

Psychologist Gordon Allport once created a list of personality traits that included more than 4,000 terms.

The question of just how many personality traits there is has been the subject of debate throughout much of psychology's history, but many psychologists today rely on what is known as the big five model of personality.

According to the big five model, personality is composed of five broad dimensions. Individual personalities may be either high, low, or somewhere in between on each of the five core traits.

The five traits that make up personality are:

  • Agreeableness: trust, altruism, kindness, affection, and other prosocial behaviors
  • Conscientiousness: high levels of thoughtfulness, good impulse control, and goal-directed behaviors
  • Extroversion: sociability, talkativeness, assertiveness, and excitability
  • Neuroticism: sadness, moodiness, emotional instability, worries about many different things
  • Openness: creative, open to trying new things, happy to think about abstract concepts

Most of the characteristics you might use to describe your own personality fall under one of these broad headings. Personality traits such as shy, outgoing, friendly, and sociable are aspects of extroversion while traits such as kind, thoughtful, organized, and ambitious would be part of the conscientiousness spectrum.

Each of these five traits represents a continuum. Some people are low in certain traits and high in others. In fact, many people are somewhere in the middle of many or most of these characteristics.

Genes vs. Environmental Causes

What matters more when it comes to personality, nature or nurture? Just how much does your DNA influence your personality? Researchers have spent decades studying family, twins, adopted children and foster families to better understand how much of personality is genetic and how much is environmental.

Both nature and nurture can play a role in personality, although a number of large-scale twin studies suggest that there is a strong genetic component.

One study, the Minnesota Study of Twins Reared Apart, studied 350 pairs of twins between 1979 and 1999. Participants included both identical and fraternal twins who were either raised together or apart. Results revealed that personalities of identical twins were similar whether they were raised in the same household or apart, suggesting that some aspects of personality are influenced by genetics.

This certainly does not mean that the environment does not play a role in shaping personality. Twin studies suggest that identical twins share approximately 50% of the same traits, while fraternal twins share only about 20%.

Personality traits are complex and research suggests that our traits are shaped by both inheritance and environmental factors. These two forces interact in a wide variety of ways to form our individual personalities.

Personality Trait Changes

When it comes to some of the broad, dominant traits, change tends to be difficult. When it does occur, these changes tend to be very subtle.

A very extroaverted person, for example, might become somewhat more reserved over time. This does not mean that they will transform into an introvert. It simply means that a subtle shift has occurred and the person's extroversion has been slightly modified. The individual is still outgoing and gregarious, but they might find that they also enjoy solitude or more quiet settings on occasion.

An introvert, on the other hand, may find themselves becoming somewhat more extroverted as they grow older. This does not mean that the individual suddenly develops a desire to be the center of attention or to spend every Friday night at a large party. However, this person may find that they begin to enjoy social events a bit more and feel less exhausted and drained after they spend time socializing.

In both of these examples, the individual's core personality trait has not changed altogether. Instead, changes over time, often the result of experiences, have led to subtle shifts in these central traits.

Key Research Principles

In their handbook of personality, Roberts and his colleagues describe a few basic principles that have been derived from personality research:

  • Identity development principle: People develop a stronger identity as they age and maturity brings a greater commitment to and maintenance of this sense of self. During the younger years of life, people are still exploring different roles and identities. As people age, they begin to feel stronger loyalty to the identity they have forged over the course of their life.
  • Maturity principle: People tend to become more agreeable, emotionally stable, and socially dominant as they grow older.
  • Plasticity principle: While personality traits tend to be stable, they are not set in stone. They are subject to environmental influences at any stage of life.
  • Role continuity principle: It is the consistency of roles that leads to continuity in personality traits rather than consistency in environments.
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  • Bouchard, T.J. Jr, Lykken, D.T., McGue, M., Segal, N.L., Tellegen, A (1990). Sources of human psychological differences: the Minnesota Study of Twins Reared Apart. Science. 1990; 250(4978):  223–228.
  • Matthews, G., Deary, I.J., & Whiteman, M. C. Personality Traits. Cambridge: Cambridge University Press; 2010.
  • Roberts, B.W., Wood, D., & Caspi, A. The development of personality traits in adulthood. In O.P. John, R.W. Robins, & L.A. Pervin (Eds.), Handbook of Personality: Theory and Research, pp. 375-399. New York: The Guilford Press; 2008.