It is well established that heredity influences personality. Although we all share 99% of the human genome, 1% is the variable genome that marks us out as individuals. Some of this variable genome influences our brains and thinking or cognitive functions. Were this not the case then our large brains and our social behaviour would not have evolved. Studies of twins suggest that identical twins, who share the same variable genome, also share 50% of the same personality traits. On the other hand, fraternal twins, whose variable genome differs, share only about 20%. We are a social species, and our personality affects our chances of survival and reproduction. It seems likely, therefore, that those genes which affect personality are subject to natural selection.
But what is personality? There are many models, all of which are simplifications, but currently the one most widely used and accepted is the Big Five Model. This comprises five traits each of which lies on a scale from low to high. They are:
- Agreeableness, which comprises pro-social behaviours such as trust, kindness, and affection;
- Conscientiousness, or a tendency to be responsible, hard-working, thoughtful, committed to goals, and to adhere to the rules;
- Extroversion, which includes stimulus seeking behaviour such as sociability and talkativeness, together with excitability and assertiveness;
- Neuroticism. A person with a high level of this trait has a tendency towards sadness, worry, and emotional instability; and
- Openness, or abstract thinking, creativity, and a willingness to try new things.
You can take a free Big 5 personality test at https://bigfive-test.com/.
It has been shown that these traits are not each influenced by a single gene but rather by many. Depression, for example, is thought to be influenced by around a thousand. The number of relevant genes that a person has affects the extent to which they may exhibit the trait. In a similar way to tosses of a coin, probability theory implies that most of us will be somewhere in the middle of the scale with just a few close to the low or high extreme.
It can be seen from their descriptions that what are referred to as “personality traits” are, in fact, behavioural traits, i.e., repetitive patterns of behaviour which characterise an individual. The focus of research has been on behaviour for the simple reason that it is observable. However, behaviour is caused by a combination of our needs, contra-needs, and beliefs about satisfiers and contra-satisfiers, some of which are unconscious. So, it is likely that heredity influences these latter causes rather than impacting directly on behaviour.
For example, heredity may influence the strengths and relative priorities of our needs and contra-needs. It may also predispose us to certain beliefs. That is, heredity does not create the belief but rather a predisposition to accept beliefs which are consistent with it. Depending on environmental factors such as upbringing, culture, role models, social learning, traumatic experiences, etc., we may or may not come to hold a belief consistent with our predisposition and, thus, display a particular personality trait.
The priorities and beliefs which affect personality form schemata. They can be established early in life and be resistant to change, but are not cast in tablets of stone. Environmental factors can influence us at any stage in life, and either alter our personalities or reinforce them.
A hereditary predisposition and an environmental trigger can also cause a personality disorder to develop. Personality disorders are repetitive patterns of behaviour which stray too far from the socially acceptable norm. In practice, this means personality traits which are unusual by virtue of being high or low on their respective scale. In turn, this means extreme beliefs about satisfiers and contra-satisfiers, unusual strengths or weakness of needs/contra-needs, or unusual ways of prioritising them. In some cases, this can lead to behaviours that are socially harmful.