Individual Differences

Personality

Evolution, Inheritance, & Personality

Last updated:
03 Apr 2006

Introduction

Variation in human characteristics as adaptive for the species

Behavioral genetics

Heritability of the big 5 personality factors

Heritability of aggression

Temperaments

A person is just a gene's way of creating another similar gene

Evolutionary theory & attraction

Evolutionary theory & mate selection

Evolutionary theory & aggression

Behavior genetics & pathology

Critique

References

 

Introduction

Animals, including humans, are born with in-built instincts to perform adaptive behaviors.  These instincts include many reflexes and relatively straightforward behaviors, such as food-seeking behavior.  But, as animal behavior gets more complex, there are in-build instincts which are correspondingly more complex (e.g., food storing behaviors).  Thus, personality in humans is considered, from an evolutionary perspective, not be qualitatively different in origin than the drive in dogs, for example, to bury their bones, or squirrels to store acorns.  It's just that human personality is even more complex.

The evolutionary perspective of personality probably makes most sense when considered in conjunction with other perspectives.  Evolutionary psychology can seen, for example, as a theoretical platform which underlies the human personality.  At birth, everyone starts from scratch, with a unique genotype, some inbuilt instincts (including a temperament), and a pre-wired capacity to learn certain kinds of behaviors.  Biological processes, psychodynamics proceses, behavioral processes, social shaping processes, etc. then unfold, interacting with the individual's genotype, to dynamically create the unique psychological characteristics of the individual.  All the time, however, this shaping occurs within certain parameters layed down by the genotype, which itself is a synthesized expression of the knowledge of human evolution about what seems to be adaptive, stored and conveyed through genetic code.

The evolutionary perspective is closely related to all other perspectives of personality.  Freud, for example, was ultimately famous for being the father of psychology, by revealing that human behavior was driven by unconscious, instinctual forces.  Freud understood personality as arising from the way in which humans were able to resolve these instinctual impulses (such as for pleasure, sex, food, etc.) with societal constraints, and the long-term needs of the individual.  Indeed, there is evidence that Freud was influenced by the writings of Darwin and that he greatly admired Darwin's work (Sulloway, 1979, cited in McAdams, 1994).

Skinner, the famous advocate for behavioral understandings of the human behavior, also understood their to be a role played by evolutionary forces, although he saw this as not being as important as environmental reinforcement: 

"We can trace a small part of human behavior...to natural selection and the evolution of the species, but the greater part of human behavior must be traced to the contingencies of reinforcement, especially to the very complex social contingencies we call cultures.  Only when we take those histories into account can we explain why people behave the way they do."
- Skinner, 1989, p. 18, cited in Feist & Feist (2002, p. 281)

Rather than see the various perspectives in opposition from one another, as Skinner seems to do here, I think it is more useful and productive to understand how they all work together in various ways to create the multi-faceted reality of human personality.

The evolutionary perspective, then, views personality as the product of a long history during which it was advantageous for humans to adopt particular characteristic ways of thinking and behaving.  Evolutionary forces are most useful for understanding some of the broad trends in apparently instinctual drives.  It is also seems that although we have been shaped as a species by the challenge of survival, understanding individual's personalities is often best approached from other perspectives, particularly because the evolutionary perspective currently seems to offer little in the way of practical intervention or assistance in dealing with personality problems.

Variation in human characteristics as adaptive for the species

An important principle of natural selection is that a species will exhibit variations in various physical and behavioral characteristics.  In this way, over time, individuals with physical and behavioral characteristics which are most adaptive for survival will be more likely to survive and pass on their characteristics to their off-spring.  Over a long period of time, this leads to eventually to entirely different species, or the gradual shaping (evolution) of a species to have some characteristics and not others.

In this light, then, observations of the wide variations in human personality can be understood as the process of evolution throwing up variations of the human psyche which allows the most adaptive personalities to survive more often and procreate.

In a complex species, such as humans, it is also important to realize that quite different personalities may prove adaptive in different ways.  For example, highly aggressive behaviors can be adapative in that they allow a person to stand up for themselves and fight for their share, or more, of available resources.  However, this also makes a person vulnerable to the aggression of others.  So, it is also understandable that more submissive or passive personalities can be adaptive.  By avoiding conflict with others, the individual is less likely to suffer direct harm from the aggression of others, but may find that it is difficult to get access to the resources for survival.

For some aspects of personality, there appear to be convincing evolutionary explanations; for other aspects, for other aspects of personality, evolutionary perspectives are less useful.  Evolutionary perspectives are probably most useful for explaining general societal behavior trends.

An example of such a trend is that males are greater perpetrators of violence than women.

During human history, it seems males evolved with particular tendencies and capacities that were advantageous for hunting and physical defence of tribes.  This underlying predisposition of males seems to also predispose males to also being more likely to have overly violent behaviors.  This may be due to higher than normal levels, for example, of particular hormones and neurochemicals (testosterone, for example).  Other behavioral sex differences which have attracted evolutionary explanations include the higher rates of promiscuity for males, and the higher rates of rape by males.

Behavioral genetics

 

Behavioural genetics studies the way inherited biological material i.e. genes, can influence patterns of behaviour.

 

Behavioural genetics has sometimes been called “trait” genetics as it examines the way our genes influence our personality traits.

 

The basic methodology of behavioural genetics is to compare similarities in personality between individuals who are and are not genetically related, or who are related to different degrees.

 

Humans are highly similar to each other genetically. About 90% of human genes are identical from one individual to another. Behavioural genetics concentrates on the approximately 10% of the human genome that does vary.

Behavioural genetics, like trait psychology, focuses exclusively on aspects of personality that differ from one individual to another. The inheritance of species-specific traits or traits that all humans share is examined later in evolutionary psychology.

The basic assumption of behavioural genetics is that if a trait is influenced by genes then it ought to be more highly correlated across pairs of identical (monozygotic:MZ) twins than across pairs of fraternal (dyzygotic:DZ) twins, and more highly correlated across closer genetic relatives than across more distant genetic relatives.

Across many personality traits the average correlation across MZ twins is .50 and across DZ twins is .30 (e.g. Bouchard & McGue, 1990; Loehlin & Nichols, 1976). Thus according to twin studies average heritability of most personality traits is .40. This is interpreted to mean that the proportion of behavioural variance that can be explained by genetic variance is 40%. (This is a heritability coefficient i.e. a percentage not a correlation coefficient).

 

Heritability of the Big Five personality factors


Are the Big 5 traits influenced by our genes? A number of twin studies have explicitly examined the heritability of C, A, O, N, and E.

  • Evidence of heritability for conscientiousness(Jang, McCrae, Angleimer, Riemann, & Livesey, 1998)

  • Evidence of heritability for agreeableness (Jang, Livesey, & Vernon, 1996)

  • Evidence of heritability for openness to experience (Loehlin, 1992)

  • Strong and consistent evidence of heritability for neuroticism

  • Strong and consistent evidence of heritability for extraversion

Graph showing high heritability for extraversion and neuroticism in identical twins.

 

Heritability of Aggression

 

Rushton, Fulker, Neale, Nias and Eysenck, (1986) found aggressiveness partially hereditary. They assessed 500+ MZ and DZ twin pairs on altruism, empathy, nurturance, assertiveness and aggressiveness and found high correlations for MZ twins for each personality variable.

 

Monozygotic twins were found to be more similar than dyzygotic twins on aggressiveness (for MZ twins, r = .40 and for DZ twins, r = .04). Their analyses indicated that nearly 50% of the variance for each personality variable was due to hereditary causes.

 

The data from this study indicate a role for genetic determination in the case of several traits related to aggression. Mednick and Volavka (1980) also found a much higher concordance rate for delinquent behaviour in monozygotic twin pairs than in dizygotic twins. In Ge et al.'s (1996) study of children adopted at birth, aggressiveness in the children was significantly related to antisocial behaviour in their biological parents. The study also found that the children's aggressive behaviour correlated with the adopted parents' parenting practices which suggests a complex interaction of environment and heredity.

Another possible explanation is that aggression in males could be linked to a chromosomal abnormality. Some males who carry an extra Y chromosome have been found to show increased aggression (ref on XYY karyotype)

The enzyme monoamine oxidase regulates the breakdown of the neurotransmitters serotonin, norepinephrine and dopamine. Low levels of MAO have been found to be associated with aggression (along with things such as extraversion and sensation seeking). There is a gene for MAO called the Monoamine oxidase-A gene. Sometimes, but very rarely, an individual may lack the MAOA gene altogether. In one very well-studied Dutch family, many of the men lack the MAOA gene altogether and the men are notoriously violent individuals.

Interestingly, there is a promoter region near the MAOA gene and this promoter area is responsible for switching the gene on and off, and thus controlling the amount of MAO. In a recent study, researchers Caspi, McClay, Moffitt and colleagues found that the combination of low-activity promoter regions together with abusive childhoods resulted in violent behaviour in adulthood. The researchers studied a large sample of more than 500 men in NZ from birth until their late 20s. Those with abusive childhoods but with a genotype which resulted in high levels of MAOA expression did not show nearly such great evidence of aggression.  This interesting study shows the complex interplay of genes and environment.

 

Temperaments

 

Buss and Plomin (1984): “Inherited personality traits evident in childhood” (p. 84):

  • temperaments genetically based

  • temperaments more pervasive in influence than other traits

  • affect what people do and how they do it

Buss and Plomin state from their extensive research that these temperaments show continuity through the lifespan BUT that they are not perfectly stable as genes don’t operate continuously but switch on and off during development AND temperament can be modified by experience.

 

Buss and Plomin identify three temperaments

  • emotionality

  • activity level

  • sociability

 

Activity Level

  • overall output of energy

  • vigour = intensity; e.g. tennis vs. chess

  • tempo = speed; prefer fast-paced activities

 

Sociability

  • desire for other people's attention, to share activities, desire for the responsiveness and stimulation that is part of social interaction

  • value interacting with others

Emotionality

  • tendency to become physiologically aroused (easily and intensely) in upsetting situations
    three emotions: distress, anger, fear (other emotions don't involve enough arousal to be relevant to this temperament)

  • the three emotions are not strongly correlated (except distress and fear)

  • similar to Eysenck's neuroticism
     

EAS temperament survey is available for adults and you’ll be doing a shortened version in the tutorial this week. It is also available for children via teacher report or parent report. Very widely used.

The three temperaments can be related to Sheldon's somatotypes:

  • activity level (mesomorph)

  • sociability (endomorph)

  • emotionality (ectomorph)

Graph:
Are Buss and Plomins 3 temperaments inherited? A number of studies have shown that the correlations were next to nothing or even inverse for DZ twins but were strong for MZ twins. Average correlations for 228 MZ twin pairs and 172 DZ twin pairs with an average age of 5 years 1 month can be seen on the screen.

Table:

The table shows correlations between adopted children and their adopted parents (AP) and adopted siblings (AS); and their biological parents (BP) and their biological siblings (BS). The measures shown are from the Thurstone Temperament Schedule. Parent correlations are averages of mother-child and father-child correlations. Sibling correlations are also averaged.
MEASURE AP AS BP BS
Sociable .02 -.13 .18 .38
Active .02 -.12 .16 .06
Vigorous .06 .18 .33 .42

Cautions:
Possible rater bias when parents rate children. There is some suggestion from research that parent ratings may be biased towards expecting MZ twins to be more similar and DZ twin parents tend to contrast the differences. Furthermore, mothers of VERY young children are not particularly good at rating their children, and we know that sometimes MZ twins may be treated in more similar fashion than DZ twins.

 

Temperaments seem to be useful descriptors of general styles, but not strongly linked to specific behaviours, etc., which probably require more detailed understanding of more specific personality traits
 

Other temperaments?
- Impulsivity?
- Intelligence?

 

A person is just a gene’s way of creating another similar gene

 

Evidence that inheritance plays a role in personality is part of the thinking that sees evolutionary processes as having an influence on human behaviour. There are a number of related disciplines such as behavioural ecology, sociobiology, evolutionary psychology, ethology and evolutionary biology. I shall be referring to this area by the generic term “evolutionary theory”.
Basic assumptions of evolutionary theory:

  • that behaviours seen in people are present because in the evolutionary history of the human species

  • these behaviours were helpful or necessary for survival

  • the more a behaviour helps us survive and reproduce ---> more likely to occur in subsequent generations.

  • The focus here is not on IDs but on patterns of behaviour common to ALL (human nature).

Evolutionary theory and attraction

 

Genetic similarity theory (Rushton, 1989; Rushton, Russell & Wells, 1984):

  • more attracted to strangers who resemble you than who don’t

  • leads to offspring with not only your genes but genes like themselves

  • Rushton’s research shows that people are more attracted to genetically similar people

  • People who go on to have children together are more alike than those who don’t. Thus among sexually active couples, those most genetically similar are those most likely to have reproduced.

  • Attraction not limited to the opposite sex. We tend to be friends with people who are genetically similar. Why? From an evolutionary perspective it’s because you are more likely to be altruistic to a friend than a stranger, making it more likely that the friend will go onto reproduce (similar genes to your own) and you may n\meet the friend’s opposite sex sibling - close genes again!

  • How do we detect this genetic similarity: either drawn to those with similar facial and bodily features OR via smell.

Evolutionary theory and mate selection

 

From an evolutionary perspective men and women are seeking the same thing:
The greatest possible likelihood of mating with someone with whom they will have healthy offspring

Male seek fertile females i.e. they are drawn to cues indicating those with good reproductive capacity (those who appear youthful and beautiful). Men seek younger mates.

Females seek economic security for their few possible offspring. They are drawn to cues indicating availability of resources, dominance (expressed in socially positive ways), and high status. Females seek older mates.

Males seek “sex objects” and females seek “success objects” according to this perspective.

Cross-cultural findings show that males and females use different behavioural strategies for finding mates. Females emphasize their fertility (e.g. youth, beauty) and males emphasize security (e.g. dominance, finance, ambition).

Note that David Buss (1989) examined mate preferences in 37 cultures around the world and found surprisingly little difference.
 

Other gender findings consistent with an evolutionary perspective:
Males are more:

interested in casual sex

relaxed in their partner criteria for one-night stands

readily excited by visual erotica 

disturbed by thoughts of females' sexual infidelity (whereas females are more disturbed by thoughts of males' emotional infidelity).

 

Example: Men and women approach sexual jealousy differently
In a study by Buss, Larsen, Westen, and Semmelroth (1992) men and women were asked to respond to the following vignette:

 

Please think of a serious committed relationship you have had in the past, that you currently have, or that you would like to have. Imagine that the person with whom you’ve become seriously involved became interested in someone else. What would distress or upset you more?

(a) Imagining your partner forming a deep emotional attachment to that person OR

(b) Imagining your partner enjoying passionate sexual intercourse with that person


FEMALES = 82% option (a) 18% option (b)
 

MALES = 40% option (a); 60% option (b)
 

From an evolutionary perspective women worry more about emotional infidelity because they are concerned about loss of support to her and her children i.e. mate will share resources with another.

 

Men worry about sexual infidelity as they are concerned that offspring might not be their own.
Why, you might ask do some women prefer men who are unstable and will treat them “bad”?

 

From an evolutionary viewpoint this doesn’t appear to make sense – or does it. According to the sexy son hypothesis proposed by Gangestad (1989) these men may leave them but if they produce a boy, he will be a “sexy son” and will leave behind numerous offspring (who will be the woman’s grandchildren).


These findings may overemphasis gender differences. There are many similarities in what both genders seek in partners.  Both genders look for:

  • good sense of humour

  • pleasing personality

  • agreeableness

  • emotional stability

  • kindness

  • lovingness

Evolutionary theory and aggression

 

According to ethology or behavioural biology (another term!) humans are members of the animal kingdom and this is emphasised as these theorists try to explain some aspects of human behaviour in terms of its primate origins.

Aggression seen to be pre-programmed to serve an adaptive purpose. Seen as a behaviour (personality trait?) that has evolved as a natural element in the behavioural repertoire of some species because it facilitates survival and adaptation to the environment.

Inter-male fighting in animals purposive and adaptive. It is likely to ensure an optimal spacing of animals within a given amount of territory (Lorenz, 1966). Aggression also allows the formation of dominance structures and thereby serves to enhance natural selection through mating of the strongest members of the species. However, for some animals spacing out is not required (e.g. horses), as there is no territorial jealousy and enough food for all (Lorenz). Nonetheless aggression still occurs and the result is the evolution of strong defenders of their young and of the herd in general (Lorenz).

 

There are however questions over whether ethology explains human aggression. Firstly, it has yet to be shown that aggressive tendencies are adaptive in humans.

 

Opponents suggest that aggression is the product of environment and learning. Evolutionary theorists don’t deny learning but insist there’s also an innate mechanism or drive.

 

Behaviour Genetics and Pathology

 

We’ll now turn our attention to a brief look at the likelihood of a genetic component to a couple of diffferent pathological conditions: schizophrenia and bipolar disorder.

 

Schizophrenia is characterized by disorientation, confusion, cognitive disturbance, separation from reality. Many studies have shown a genetic susceptibility to schizophrenia exists. For example, it has been shown that in general, parents brothers and sisters of schizophrenics are more likely to suffer from schizophrenia than those not so related. In fact, the more closely one is related to a schizophrenic, the greater the likelihood of having schizophrenia.

 

In a study conducted by Gottesman and Shields (1972) a concordance rate of 50% in MZ twin pairs and 9% in DZ twin pairs was found.
 

In a well-known study conducted in Denmark with 5000 children of schizophrenic parents who have been adopted or reared away from their parents (Kety et al., 1975) it was found that the biological relatives of schizophrenics were significantly more often schizophrenic than the biological relatives of a control group matched for age, sex and social class. The adopted relatives of the schizophrenics (i.e. the adoptive parents, siblings etc) showed no greater incidence of schizophrenia than the controls.
It has also been found that the MZ twin of a schizophrenic is not only more likely to be schizophrenic but is also more likely to show other forms of psychosis even if not diagnosed with schizophrenia (Heston, 1970).

It is thought likely that a diathesis-stress model best explains schizophrenia. I.e. people have a genetic diathesis or predisposition towards the illness and then environmental factors influence whether the disease is realized.

Bipolar disorder (previously manic-depression): characterised by severe depressions, alternating with hyperactive, frenetic, talkative behaviour, grandiose thoughts, a rush of ideas (mod may be elevated but can also be highly agitated).

 

Twin studies reveal genetic component (e.g. Tsuang & Faraone, 1990)
one study found link with specific gene on chromosome 11 in a group of Amish families but not found in other studies - so that gene not always responsible.


Twin studies also find evidence for genetic links for:

  • alcoholism (Eysenck, 1964) MZ twin concordance 65%; DZ twin concordance 30%: remember these are adults --> environmental factors? However, other eresearch has found the same.

  • antisocial behaviour (Eysenck, 1964) Higher concordance rate among MZ twin pairs than DZ twin pairs. Other research in agreement.

Critique

 

Heritability coefficients are not nature-nurture ratios. They don’t necessarily always tell you how much a trait is determined by genes as opposed to environment. For example, television watching has been found to be heritable to a significant degree (Plomin, et al., 1990). So do we have a gene for TV watching?


More likely to some basic propensity that has some genetic component:

lethargy?
sensation seeking?
craving for blue light?
 

Combine with early environmental experiences = propensity to watch lots of TV

Evolutionary theory:
Because a plausible theory can be constructed about how a behavioral pattern may have been evolutionarily adaptive, it does not necessarily follow that the evolutionary needs forced the behaviour pattern

 

None of these things tell you how personality develops, at best they can tell you that genes are involved SOMEHOW!

References

Burger, J. M. (1993). Personality (3rd ed.) Pacific Grove, CA: Brooks/Cole.

Carver, C.S., & Scheier, M.F. (2000). Perspectives on Personality (4th ed.) Needham Heights, MA: Simon & Schuster.

Feist, J., & Feist, G. J. (2002). Theories of personality (5th ed.). New York: McGraw-Hill.

Funder, D. C. (1997). The personality puzzle. New York: W. W. Norton.

McAdams, D. P. (1994). The person: An introduction to personality psychology (2nd ed.). Orlando, FL: Harcourt Brace.

Ornstein, R. (1993). The Roots of the Self: Unraveling the mystery of who we are. New York: HarperSanFrancisco.

Phares, J.E. (1991). Introduction to Personality (3rd ed.). New York: Harper Collins.

Ridley, M. (1999). Genome: The autobiography of a species in 23 chapters. London: Fourth Estate.