Evolution, Inheritance, & Personality
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:
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.
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.
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.
Graph showing high heritability for extraversion and neuroticism in identical twins.
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.
Buss and Plomin (1984): “Inherited personality traits evident in childhood” (p. 84):
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
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 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
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
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”.
Genetic similarity theory (Rushton, 1989; Rushton, Russell & Wells, 1984):
From an evolutionary perspective men and women are seeking the same
Other gender findings consistent with an evolutionary perspective:
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
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
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.
about sexual infidelity as they are concerned that offspring might not
be their own.
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).
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
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.
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
Twin studies reveal genetic component (e.g. Tsuang & Faraone, 1990)
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?
Combine with early environmental experiences = propensity to watch lots
None of these things tell you how personality develops, at best they can tell you that genes are involved SOMEHOW!
Burger, J. M. (1993). Personality (3rd ed.) Pacific Grove, CA:
Ornstein, R. (1993). The Roots of the Self: Unraveling the mystery of
who we are. New York: HarperSanFrancisco.