The Genetic Basis of Behaviour & Evolution (AQA A Level Psychology): Revision Note

Assumptions of the biological approach

• The biological approach (BA) assumes all psychological behaviours/functions have a biological root at their core

• To understand human behaviour it is necessary to look at the biological:

  • structures

  • functions

  • processes

• Examples of biological structures, functions and processes include

  • Genetics e.g. the role of inherited traits in behaviour

  • Neurotransmitters e.g. the role of serotonin in behaviour

  • Hormones e.g. the role of testosterone in behaviour

  • Brain structures e.g. the role of the hippocampus in behaviour

• The BA assumes a biological basis for behaviour e.g.

  • memory is a function of the hippocampus

  • depression is the result of irregular neurotransmission

  • aggression is a product of excess testosterone

The genetic basis of behaviour

• The genetic basis of behaviour investigates the extent to which behaviours are inherited

  • People inherit physical characteristics, such as natural hair and eye colour from their parents, but there are still questions about whether people inherit psychological traits such as competitiveness, aggression, and sensitivity

  • A genetic explanation of behaviour is that genes influence all traits, characteristics and behaviours e.g. intelligence is inherited in the same way as height and other physical characteristics

• Typically each individual is born with 23 pairs of chromosomes which form the individual's genotype

  • An individual's chromosomes make up their DNA, which is essentially a set of 'instructions' determining individual characteristics

• Twin studies are used to investigate a genetic basis for specific traits e.g. schizophrenia, IQ, criminality

  • Monozygotic (MZ) twins are compared to dizygotic (DZ) twins

  • A twin study begins by looking at one twin proband; if this twin has the specific trait/behaviour (e.g., schizophrenia), the researchers then see if their twin sibling also possesses that trait/behaviour

  • Concordance rates are used to measure the rates of agreement in a twin study

    • A high concordance rate indicates evidence of a causal connection between the twin type and the trait/behaviour

• If a characteristic is genetic, it would be expected that 100% of MZ twins would share that characteristic, as they share 100% of their DNA  

• McGuffin et al. (1996) found a concordance rate of 46% for major depression with MZ twins; for DZ twins the rate was 20%

  • This finding provides evidence that there is a strong genetic basis for depression, yet it cannot be said that depression is fully a genetic trait

Genotype & phenotype

• Genotype refers to a person's particular set of genes that make up their DNA

  • It is made up of a combination of alleles that comprise a specific gene

• Everyone has a unique genotype (apart from MZ twins, who share 100% of their DNA)

• Genotype determines physical characteristics such as eye and natural hair colour 

• Genotype is set/fixed at the point of conception i.e., it can't be changed (unless a specific, spontaneous - and very rare - genetic mutation occurs)

• One genotype can produce a wide range of phenotypes depending on the environmental factors present

• Phenotype refers to the expression of the genotype determined by prevailing external influences and comprises an individual's observable traits, e.g.,

  • height

  • eye colour

  • hair texture

  • blood type

Environmental factors

• Environmental factors, such as nutrition or exposure to pollutants, can influence a person's phenotype e.g.,

  • An individual's genotype will determine how tall they could potentially grow; however, their diet in childhood will determine whether or not they reach their maximum height 

• A person's genotype may predispose them to specific behaviours, e.g., criminality but if specific environmental factors are not in place, then this aspect of their genotype may never be expressed in their phenotype

  • If someone with potential criminality in their genotype has a secure home, a good education, a good income and happy relationships, they are unlikely to turn to crime

  • If someone with potential criminality lives in adverse social conditions, then they are more likely to turn to crime

• Even MZ twins will present different phenotypes depending on their upbringing, friendship groups, hobbies, careers, etc.

Evolution & behaviour

• Evolutionary psychology is part of the biological approach to explaining behaviour (based on Darwin’s theory)

• The evolutionary approach to explaining behaviour looks at how and why specific human traits have evolved via the process of natural selection, e.g.,

  • Aggression

    • evolutionary usefulness to fight off predators and enemies

  • Memory

    • evolutionary usefulness to recall the location of food sources

  • Language

    • evolutionary usefulness to communicate with tribe members

  • Mate selection

    • evolutionary usefulness to select a mate who will bear healthy offspring

• Due to natural selection, behaviours and traits which are useful for survival and reproduction are retained, and those which are not eventually die out

Adaptive behaviours

• Adaptive behaviours are those which increase the chances of survival and reproductive success

  • This is why these behaviours are inherited through the generations, i.e., how we behave now is the product of causes rooted in the distant past

    • This is known as the ultimate causes theory of behaviour

• Genes are the mechanisms which drive evolution

  • Any genetically determined behaviour which increases the chances of survival per species will be inherited to future generations

• Biological psychologists believe the characteristic of intelligence is adaptive as it enhances human survival

  • For our ancestors, intelligence manifested as knowing where to find the best food source, understanding weather patterns, fashioning tools, creating transport, negotiating with other tribes etc.

  • Intelligence today equates to further education, training, an entrepreneurial drive, a good job

    • These accomplishments ensure security and abundance of resources, i.e. survival and reproductive success

Evaluation of the genetic basis of behaviour

Strengths

• The biological approach has a range of real-world applications, e.g.,

  • the success of drug therapies such as SSRIs for the treatment of psychological disorders such as depression and OCD

  • family history is a reliable predictor of an individual’s disease risk due to an understanding of genotype combined with environmental factors

• Twin studies tend to use large samples, e.g.

  • Bouchard et al. (1990) conducted a large-scale study into the heritability of IQ

  • They collected data on over 1,500 pairs of twins, as well as over 350 adopted and biological siblings

  • MZ twins reared together showed a correlation of +0.9 (a hugely significant finding)

    • The large sample size and quantitative data give this study good reliability

Limitations

• Arguing that complex human behaviour is a product of genetics or evolution alone is an example of biological determinism

  • This is a simplistic view which ignores the effect of environmental factors on behaviour

  • People are capable of sophisticated cognitive processing, which is not acknowledged by the biological approach

    • This limits the external validity of the approach

• Some well-known twin studies were conducted before the use of DNA testing to determine zygosity

  • If a pair of twins closely physically resembled each other they might have been labelled MZ when in fact they were DZ

  • Any question as to the veracity of a twin's MZ/DZ status would damage the validity of twin study findings