PSYCH – Week 19 Online Readings
Week 19: Heritability and Evolutionary Psychology
Focus Question: What are the evolutionary underpinnings of behaviour?
Introduction – Genetics Review
What is the relationship between genes, heredity, and behaviour?
Our genetic material, or DNA, is organized into structures called chromosomes that are
located in the nucleus of every cell. Genes can be defined as regions of chromosomes that
encode particular proteins.
Evolutionary psychology: Evolutionary psychology is concerned with the evolutionary
underpinnings of behaviour – understanding the adaptive significance and utility of
behaviours exhibited by modern humans.
Studies find that identical twins show a greater correspondence in many traits than
do non-identical siblings. However, these are less than perfect as it is sometimes
difficult to separate genetic influence from environmental influences. The same
genes can be expressed very differently
Genetic influence is measured by a statistic called the heritability coefficient, or
h . Heritability is the proportion of observed variance in a behaviour that can be
attributed to genetic differences among individuals. Thus, a trait that shows high
heritability necessarily varies within a population.
Until recently, investigations of the genetics of behavioural disorders have been
guided by the one gene, one disorder (OGOD) hypothesis. This approach has
led to the discovery of the gene for Huntington’s chorea.
The pedigree studies (following of several generations of families) and the OGOD
strategy have not been successful in the area of major mental disorders. This is
because the etiology of major mental disorders does not involve single genes of
large effect. Instead, many genes are involved; they are polygenic.
Alleles, not Pedigrees
The search for many genes of small effect has led to the increasing use of allelic
association designs (recall that alleles are alternative forms of the same gene).
These studies compare affected and unaffected individuals, regardless of their
kinship status. The Human Genome Project has identified a large number of
markers on many chromosomes. The identification of these markers means that
association studies can identify genes of modest effect. This improvement allows
pedigree studies to be bypassed and, because association studies are easier to
conduct, will contribute to progress in the study of polygenic disorders and traits
such as intelligence and personality.
Genes that do not reproduce cease to exist. Therefore, the commonality among
genes that exist is that they can reproduce themselves.An organism is simply a way for genes to reproduce themselves. The gene that increases chances of
survival should become more common in subsequent generations.
Would you expect higher heritability estimates in a heterogeneous population or a
If the population is homogenous with regard to the environment, heritability estimates
will be higher; if it is genetically homogenous, heritability estimates will be lower.A
homogenous environment can lead to higher h , but a genetically homogeneous
population leads to lower h .Apopulation in an environment homogenous on both
accounts, such as Scandinavia, will likely produce higher heritability estimates than a
population in a heterogeneous environment, such as the United States, because there is a
larger selection in the gene pool to draw from and higher genetic variance. The larger
selection provides more opportunity for different gene combinations.
There is a relationship between evolutionary change and behavioural genetics.
Changes in evolutionary fitness require additive genetic variance, genetic variance due to
gene dosage, as opposed to interactions among different genes (non-additive variance)
and is responsible for the resemblance between parents and offspring. If additive genetic
variance for fitness-related characteristics is present in a species, then there is opportunity
for further selection. When the population reaches maximal fitness, the additive genetic
variance is zero. The genes are said to have gone to fixation.
Evolutionary fitness: The probability that the line of descent from an individual with a
specific trait will not die out.
Heritability statistics can be computed using contrasts between identical
(monozygotic or MZ) and fraternal (dizygotic or DZ) twins or between genetically
related individuals reared together (T) and apart (A). Because MZA(monozygotic, reared
apart) twins share identical genetic material and no common environment, the
correlations of MZAtwins directly reflect heritability.
Environmental influence can be partitioned into shared environment (aspects of
the environment shared by all family members, such as household socioeconomic status)
and non-shared environment (aspects of the environment not shared by all members, such
as differential parental treatment of siblings). The difference in correlation between MZA
and MZT twins directly estimates the effects of shared environment.
Variations in Shared Factors
Non-shared environment has important effects on traits such as intelligence and
personality, but shared environment does not.
Human nature: Range of human behaviours that differentiate mankind from
other species, made up of adaptations, which are all behavioural characteristics that are
the product of natural selection in ancestral environments.
Behaviour has Two Interrelated Causes
1) Structure and chemistry of the organism.
a. Genetically determined, result from selective pressures of ancestral
2) Environment Evolutionary psychology Concerned with the design of human nature, all human
behavioural characteristics that are the product of natural
selection in ancestral environments.
Behavioural genetics Concerned with the partition of individual differences into
genetic and environmental variance components.
Developmental Concerned with the environmental variance component, that
psychology is, the prenatal and postnatal environmental influences
affecting individuals during their lifetimes.
Evolution by natural selection consists of descent with modification through the
differential survival of offspring. Genes that code for morphological, physiological,
neuronal, and behvaiorual charactritics associated with relative reproductive success
become more frequent over generations.
Misconceptions about Evolution
• Evolution has no foresight and humans are not more highly evolved than other
animals. Evolution is not guided.
• Aspecific allele may create a different genetic load at different times or in
• Naturalistic fallacy: Determination of what should be based on what is natural;
whatever is natural cannot be wrong and we must accept things as they are.
o Selectionist : The belief that natural selection is the fundamental
factor in evolution.
• Genetic deterministic fallacy: the belief that genes determine behaviour
independently of environmental influences. (Truth:All characteristics that have
been selected by evolution are expressed in an environment and are often
introduced by environmental context.)
Genetic load: The reduction in overall evolutionary fitness for a population compared to
what the population would have if all individuals had the most favoured genotype.
Genetics: Natural Selection
Behaviours that are determined by the immediate environment are termed facultative
(e.g., any cross-cultural differences in parenting or a collectivist vs. individualist
orientation), as opposed to obligate behaviours (e.g., the experience of sexual attraction,
regardless of where it is directed), which develop to a large degree independently of
variations in environmental context.
The pain an individual feels when he falls down is obligate, while the pain a child feels
when her mother and father divorce is facultative.
Evolutionary theories are environmental and selectionist in orientation because past
environments are posited to have selected characteristics of organisms by acting at the
level of individual genes. However, it is important to understand that evolution has no
foresight, it is not guided, and humans are not more highly evolved than other animals.
Characteristics that have been favoured by evolution are not morally good or bad.All
characteristics that have been produced by evolution need an environment to be
expressed in and are often influenced by environmental context. Epigenetics
Researchers have recently documented changes in cellular inheritance that are not
caused by changes in DNA
There have been documented cases of epigenetic modification – changes in
cellular inheritance not due to changes in the DNAsequence. These can occur
because of environmental experience; this raises the very controversial possibility
that an acquired characteristic could be inherited by offspring.
If this proves to be true, many evolutionary theories must be reassessed.
Some of the most convincing past evidence for evolution came from maladaptive
traits, such as in human the air and food passing through throat (choking hazard).
Adaptations have historically contributed to the reproductive success (fitness) of
individuals or to the reproductive success of individuals and their relatives
Theory (Daly and Wilson) – our perceptions of self-interest evolved as a way to
indicate expected gains and losses of fitness. In this view, evolution designed
people to desire things and experience emotions that increased their fitness in
Sexual preference (for males) appears to be an adaptation, as it meets criteria.
Adaption must meet the following four criteria:
1. It is obviously designed to accomplish some biological purpose.
2. It operates in a similar manner over cultures and time.
3. It is plausibly related to reproductive and survival success in ancestral
4. It is not more simply explained on other grounds (for example, as a by-product of
another characteristic or adaptation, or as pathology).
Parental Investment Theory
Parental investment theory: The energy, time, resources, and opportunity cost
associated with producing offspring.
Cornerstone theory for human mating
Parental investment cost contrast with mating opportunity cost, the effort and
costs incurred in securing and preserving mating opportunities. Sex differences in
the costs associated with parental investment create different life histories for the
sexes through a few simple rules. Selection Preferences
The sex that has the higher potential reproductive rate is under greater selection
pressure to compete, directly or indirectly, for access to members of the other sex.
o This sex often develops sex-specific morphological structures such as
antlers, etc, and behavioural tendencies.
o The sex that has lower minimum parenting costs has the higher potential
reproductive rate, and vice versa.
o Life histories are not dependent on which sex incurs the larger minimal
parental investment. When females incur the larger investment, as in all
primates, they are more selective about sexual partners, less competitive
among themselves than males are, and experience smaller variations in
reproductive success. When females incur the smaller investment, as in
some species of seahorses, they fight for access to males, are less
discriminating than males are, and experience larger variations in
Females to Males Ratio – Equal
One modern theory (originally proposed by Ronald Fisher) asserts that the ratio of
parental investment in the two sexes should be equal. Because each offspring is
the product of a male and a female, members of the rarer sex will produce more
offspring than members of the common sex. Members of the rarer sex, therefore,
will be more reproductively successful than members of the more common sex,
resulting in an equal sex ratio at equilibrium.
In mammals, females invest more in offspring than males do because of lactation and
internal gestation. Thus, mammals tend to exhibit polygyny to varying degrees. Polygyny
means that some males have multiple female reproductive partners and some have very
few or none at all.
Somewhat paradoxically, when males of a species has the ability to achieve great
reproductive success with many females, many males of that species have complete
reproductive failure and no offspring at all.
The degree of polygyny in a species is often associated with greater male size, as
illustrated by the marked dimorphism of elephant seals. This greater male size can be
explained by intermale competition for females. The different size of human males and
females and other evidence implies moderate polygyny in the human ancestral
Risks and Rewards – Behaviour of the Sexes
Males more likely to engage in risk-taking behaviour because of greater