3.1—genetic and evolutionary perspectives on behaviour

Question 1

3.1 Learning Objectives

Answer 1

• know the key terminology related to genes, inheritance, and evolutionary psychology.
• understand how twin and adoption studies reveal relationships between genes and behaviour.
  
  • twin studies typically compare monozygotic twins and dizygotic twins to estimate heritability.
  • adoption studies allow researchers to determine heritability:
  • a number between 0 and 1.0 that estimates the degree to which individual differences in a trait (in a given population) are due to genetic factors.
  • a heritability of 1.0 would mean that genes contribute to 100% of individual differences.
  • many human characteristics, including intelligence and personality, have heritability estimates ranging between 0.40 and 0.70.
• apply your knowledge of genes and behaviour to hypothesize why a trait might be adaptive.
• analyze claims that scientists have located a specific gene that controls a single trait or behaviour.
  
  • most psychological traits, as well as disorders such as Alzheimer’s disease, involve multiple genes, some of which may not even yet be discovered.
• analyze explanations for cognitive gender differences that are rooted in genetics.
  
  • males with high testosterone levels are associated with better performance on a mental rotation task, and the male advantage has been found cross-culturally.
  • it seems plausible that this difference has a genetic basis.

Question 2

3.1 Focus Questions

Answer 2

• how is human behaviour influenced by genetic factors?
• how do nature and nurture interact to produce behaviour?
• how has evolution played a role in modern-day human behaviour?

Question 3

Genes

Answer 3

the basic units of heredity; genes are responsible for guiding the process of creating the proteins that make up our physical structures and regulate development and physiological processes throughout the lifespan.

Question 4

DNA

Answer 4

• DNA (deoxyribonucleic acid): a molecule formed in a double-helix shape that contains four nucleotides. (figure 3.1)
• made up of sequences of nucleotides: A (adenine), T (thymine), C (cytosine), and G (guanine).

Question 5

Genotype

Answer 5

the genetic makeup of an organism—the unique sets of genes that comprise that individual’s genetic code.

Question 6

Phenotype

Answer 6

the physical traits or behavioural characteristics that show genetic variation, such as eye colour, the shape and size of facial features, and even personality.

Question 7

Chromosomes

Answer 7

• chromosomes: structures in the cellular nucleus that are lined with all of the genes an individual inherits.
• trisomy: when an individual has three chromosomes, thus altering the genetic make-up as well as a phenotype of the individual; the most common is trisomy 21, which is Down Syndrome.
• homozygous: when two corresponding genes at a given location on a pair of chromosomes are the same.
• heterozygous: when two genes differ.

Question 8

Tasting PTC

Answer 8

• our ability to taste a bitter substance called phenylthiocarbamide (PTC) is based on which combination of genes we inherit from either parent.
• the test for whether you can taste PTC (the phenotype) is performed by placing a small tab of paper soaked with the substance on your tongue.
• those who are tasters inherit at least one copy of the dominant gene (T) for tasting from either parent.
• people can also inherit a recessive copy of this gene (t).
• those who can taste air either homozygous dominant (TT) or heterozygous (Tt).
• non-tasters are homozygous recessive (tt).

Question 9

Behavioural Genomics

Answer 9

• behavioural genomics: the study of DNA and the ways in which specific genes are related to behaviour.
• The Human Genome Project: a massive effort to identify the components of the entire human genome; completed in 2003 and resulted in the identification of approximately 20-25k genes.
• the project led to an abundance of new techniques and information about where genes are located.
• researchers can now compare the genotypes of different groups of people to look for differences that might shed light on the cause of different conditions.
• behavioural genomics is limited in that even though a single gene has been identified as a risk factor for a disease, not everyone who inherits the gene develops the disease.

Question 10

Single Genes and Behaviour

Answer 10

• typically, combinations of genes influence behaviour.
• a single gene most likely affects more than one trait.
• when you encounter a headline beginning “Scientists find gene for…,” don’t read it as “Scientists found the gene for….”

Question 11

Behavioural Genetics

Answer 11

• behavioural genetics: the study of how genes and the environment influence behaviour.
• behavioural genetic methods applied to humans typically involve comparing people of different levels of relatedness, such as parents and their offspring, siblings, and unrelated individuals, and measuring resemblances for a specific trait of interest.

Question 12

Monozygotic Twins

Answer 12

(identical twins) come from a single ovum (egg), which makes them genetically identical (almost 100% genetic similarity).

Question 13

Dizygotic Twins

Answer 13

(fraternal twins) come from two separate eggs fertilized by two different sperm cells that share the same womb; these twins have approximately 50% of their genetics in common.

Question 14

Twin and Adoption Studies

Answer 14

• the assumption underlying studying twins is that if a trait is genetically determined, then individuals with a greater genetic similarity will also have a greater similarity for that trait.
• studying adopted children represents the nurture side of the continuum.
• if adopted children are more like their adopted parents than their adoptive parents on measures of traits such as personality and intelligence, we might conclude that these traits have a strong genetic component.
• if the children are more like their adoptive, genetically unrelated parents, a strong case can be made that environmental factors outweigh the biological predispositions.
• young adopted children are more similar to their adoptive parents in intelligence levels than their biological parents.
• by the time they reach 16, adopted adolescents score more similarly to their biological parents in tests of intelligence, suggesting that some genes related to intelligence do not exert their influence(s) on behaviour until later on in development.
• longitudinal studies: studies that follow the same individuals for many years, often decades.

Question 15

Heritability

Answer 15

• heritability: a statistic, expressed as a number between zero and one, that represents the degree to which genetic differences between individuals contribute to individual differences in a behaviour or trait found in a population.
• a heritability of 0 means that genes do not contribute to individual differences in a trait.
• a heritability of 1.0 indicates that genes account for all individual differences in a trait.
• heritability scores do not simply reflect how much genetics contributes to the trait itself, but the degree to which genetics explain differences between people with the trait.
  
  • for example, the heritability of having a mouth is 0; this isn’t to say that genetics has nothing to do with us having mouths, but we all have one, so there aren’t many individual differences to deal with for this trait.
• heritability estimates are rarely a value of 0 or 1.0.
• these estimates change as we age, as the environment never stops interacting with our genes.
• any estimate of heritability is affected by (1) the amount of genetic variability within the group being studied and (2) the variability in the environments that members of that group might be exposed to.
• what heritability estimates don’t tell us is how that interaction occurs in our bodies and brains.
• what heritability estimates don’t tell us is how that interaction occurs in our bodies and brains.
• of the 20-25k genes in the human genome, around 6-7k are active in the human brain.
• these genes influence the development of different brain structures, the production of chemicals that allow brain cells to communicate with each other, and the refinement of connections between cells that allow large-scale brain networks to form.
• if some genes fail to be expressed properly, people may be at a greater risk for developing brain-related disorders.

Question 17

Epigenetics

Answer 17

• epigenetics: the study of changes in gene expression that occurs as a result of experience and that do not alter the genetic code.
• studies of mice have shown that increased maternal licking and grooming (the rodent equivalent of cuddling) led to an increase in the expression of the GR gene in the hippocampus.
• low levels of licking and grooming led to decreased GR expression and larger stress response.
• decrease GR expression was noted in a study of human childhood abuse victims, who later committed suicide.
• this demonstrates the power of these gene-environment interactions.

Question 18

Serotonin

Answer 18

• serotonin is related to mood, and imbalances are associated with depression. (figure 3.4)
• there are two versions of the gene related to the release of serotonin—short and long.
• people who inherit two copies of the short version are at greater risk for developing depression.
• those who inherit two long copies are at a far lower risk.
• people who are heterozygous show intermediate responses to stressful events.
• the gene-environment interaction becomes apparent after an accumulation of events.
• diathesis-stress model of psychological disorders: the interaction between a genetic predisposition for a disorder and life stress.

Question 19

Natural Selection

Answer 19

the process by which favorable traits become increasingly common in a population of interbreeding individuals, while traits that are unfavorable become less common. (figure 3.5)

Question 20

Evolution

Answer 20

• evolution: the change in the frequency of genes occurring in an interbreeding population over generations.
• evolution is not a continuous process; if an animal is perfectly adapted for its environment, then there’s no evolutionary pressure for change to occur.
• any modern species is based upon version after version after version of species that were fit for their particular environment and time.
• evolutionary psychology: a modern branch of psychology that attempts to explain human behaviours based on the beneficial function(s) they may have served in our species’ development.

Question 21

Intra-Sexual Selection

Answer 21

• intra-sexual selection: a situation in which members of the same sex compete in order to win the opportunity to mate with members of the opposite sex.
• e.g. rutting: violent fights in which some deer and caribou lock horns; the winner is much more likely to mate with females than is the loser.
• e.g. alpha males: males who are dominant intimidate other males and is more likely to mate with multiple females than are the subordinate males.
• intra-sexual selection is evolutionarily advantageous because the animals most likely to become more dominant are the strongest and/or smartest, and therefore the most fit for that time and place.

Question 22

Intersexual Selection

Answer 22

• intersexual selection: a situation in which members of one sex select a mating partner based on their desirable traits.some of these traits are a result of genetics.
• e.g. a number of experiments have shown that people rate symmetrical faces as being more attractive than asymmetrical faces.
• not all elements of intersexual selection are the gift (or curse) of our genes.
• e.g. to hint at their masculinity, men often wear clothes that display their muscles and attempt to appear large and athletic, particularly when around potential mates.
• e.g. women tend to dress more attractively when they’re ovulating.

Question 23

The Colour Red and Sexiness

Answer 23

• women who are wearing red clothes are perceived to be more interested in having sex than women wearing other colours.
• this perception occurs even when the same woman is shown in identical t-shirts whose colours has been digitally altered; it is not affected by the attractiveness of the female model.
• red is associated with sexual receptivity in many animals—female baboons and chimpanzees have redder chests and genitals when they are near ovulation than at other times of their cycles.
• in humans, sexual interest is associated with flushing in the face, neck, and upper chest.
• evolutionary psychology explanations are often difficult to definitively prove; it’s difficult to separate the evolutionary forces from learned associations.

Question 24

Why the Brain is So Powerful

Answer 24

• human brains have more folds and grooves on the outer surface of the brain than any other species; this tissue is the cerebral cortex.
• the front third of the human brain is more developed than that of any other species; this area is known as the frontal lobes, and is critical for our ability to form plans, solve problems, make decisions, and control our attention and actions, among many other functions. (figure 3.8)
• neotony: the human brain does not finish developing at birth; rather, the sections of the skull are not fully fused, allowing more brain growth to occur.
  
  • this has allowed humans to develop large brains while limiting (to some degree) the physical dangers associated with the baby exiting the narrow birth canal.
  • without neotony, humans would either have smaller brains, or females would need to change their body shape.

Question 25

Evolution and Cognition

Answer 25

• the hunter-gatherer theory of sex differences in cognition explicitly links performance on specific tasks to the different roles performed by males and females over the course of our evolutionary history.
• males’ responsibilities would favour individuals with good spatial skills; females’ responsibilities would favour memory for the location of objects.
• it is important to be careful about over-interpreting the findings.
• although males and females differ on some skills, the differences are generally quite small.
• there are often alternative explanations for the results.