Critically consider the balance of genetic and environmental influences on human behaviour [22]

What is genetic Influence? (3) - is referred to as “nature”. This does not simply refer to abilities present at birth but to any ability determined by genes, including those that appear though maturations.

What is Environmental Influence? (3) – Environmental influences are referred to as nurture and are learned through interactions with the environment, which includes both the physical and the social world, and may be more widely referred to as experience. 

Research has uncovered a new understanding of genes, called epigenetics (i.e. “on top of genes”) this refers to the material in each cell of your body that acts like a set of “switches” that turn genes on or off. Life experiences, like nutrition and stress, control these switches and most importantly, these “switches” are passes on to subsequent generations.

 This explains why cloning doesn’t produce identical copies. Cloning involves placing the genetic material from one individual into an egg that had no nucleus. The egg should then grow into an identical copy of the cloned individual, but that does not happen. The reason is that there is epigenetic material in the donor egg cell, this epigenetic material was produced by environmental effects in the donor’s lifetime.

What does this mean? It means that genetics and environment are much less separate than was previously thought. 

On the one side, “nature” has been linked with twentieth-century eugenics (enforced selective breeding), as advocated by the Nazis. If nature determines behaviour then the human stock can be improved by selective breeding. Many other countries (particularly the US) sterilized thousands deemed “inadequate”. On the other side, the “environment is all” idea is popular with people who believe that any human trait can be altered with the appropriate changes in social institutions. This had led to equally brutal regimes such as Stalin (in the former Soviet Union) and Pol Pot (in Cambodia) who believed that you could engineer the behaviour of others through conditioning so that they behaved as you wished then to behave. Such political agendas may mean that arguments for or against nature or nurture, may be as much founded on political beliefs as on scientific evidence (Pinker, 2003) 

Genes modify the environment- indirect effects; genes may affect behavior directly e.g. a gene for schizophrenia creates vulnerability for the disorder. Plomin et al. 1977 suggests the following indirect effect on genes.

1)   Reactive influence- genetic factors create an infant’s microenvironment. E.g. a child who is genetically more aggressive provokes an aggressive response in others. This response becomes part of the child’s environment and affects the child’s development.
2)   Passive influence- Parent’s genes determine aspects of their behaviour. For example, a parent with a genetically determined mental illness creates an unsettled home environment. In this case the child’s mental disorder may be due to indirect, passive effects.
3)   Active influence- Or what Scarr and McCartney (1983) call niche picking. As children grow older they seek out experiences and environments that suit their genes. This explains why the influence of genes increases the influence of shared environment gradually disappears with age. The importance of the term “shared environment” is the understanding that what was once regarded as “ the environment” breaks down into surroundings that are shared with peers and siblings (such as teachers or parents or books) and those that are unshared (such as your particular friend or the age when your parents divorced). Some aspects of this unshared environment are actively selected (or “picked”) by an individual and, most importantly it is your genes that influence the kind of things you prefer- your selections.   

One argument that states behaviour is determined by genetic influence suggests that there are genetic explanations for many psychological conditions. For example, there is evidence that schizophrenics have a fault in the gene PPP3CC, the closer person is genetically to someone with schizophrenia the more likely they are to develop the disorder, in fact those with a relative are 18 times more likely to develop it.  Therefore this also suggest strong evidence for genetic influence especially as this evidence tends to be very well replicated and supported by twin, adoption and family studies time and time again. One weakness is that disorders interestingly also has behavioural and social explanations. For example Freud suggested having the experience of uncaring parents can lead to regression of the ego as an ego defence causing schizophrenia. 
Regarding intelligence, Twin and adoption studies suggest that a large component of the variation in IQ is due to genetic factors. Other evidence comes from gene mapping studies (e.g. Chorney et al 1998) where individual genes associated with high IQ have been identified.However… There is equally strong evidence for the effects of nurture, for example the Flynn effect is the observation that IQs all over the world have increased as much as 20 points in 30 years, which must be due to environmental enrichment. 

One argument that states behaviour is determined by environmental influence suggests there are environmental explanations of various psychological conditions. For example there is evidence that aggressive behaviour is mainly learned through observation and reinforcement. Bandura suggested that this enables us to learn specifically how to perform aggressive acts, how often to be aggressive and when to act aggressive.

Therefore suggesting strong evidence for the role of environment in the development of behaviours.  However one weakness is that it excludes the role of genetics, which Coccaro et al found explained nearly 50% of thee variation in aggressive behaviour in twins’ pairs. 

One argument that states behaviour is determined by genetic influence suggests that a number of approaches and theories support the genetic view. For example Lombroso and Sheldon both believe that a person’s genetic makeup determined whether they would engage in criminal behaviour or not. Lombroso believed criminals had a prehistoric man, whereas Sheldon believed a person’s body shape was responsible for their criminal behaviour. Therefore there is a strong support from the approaches for the argument that genetic influences are the cause for all behaviour.However one weakness is that the best explanations come from multi-dimensional approaches and theories, suggesting any explanation that takes into account just one element is likely to be flawed. 
Regarding evolutionary explanations, any evolutionary explanation is based on the principle that a behaviour or characteristic that promotes survival and reproduction will be naturally selected because it is adaptive and thus the genes for that behaviour will be passed on to subsequent generations. Study by Buss (1989) illustrated evolutionary pressures. He showed that certain aspects of mate choice behaviour are universal, such as the tendency for women to value ambition and industriousness more than males. The fact that such behaviours are universal implies there must be a genetic basis.

However… Such genetic behaviours are modified by culture. Buss found exceptions to the general tendencies, for example in 22% of the samples females did not value ambition and industriousness more than males. This is described as cultural relativism. 

One argument that states behaviour is determined by environmental influence there is more approaches support environmental influence than genetic influence. For example, the behaviourist theory suggest schizophrenia is due to a lack of stimulation and interaction in early childhood, causing odd behaviour, leading to attention which reinforced the odd behaviour leading them to be labelled schizophrenic.Therefore it appears that there is more acknowledgement of the role of the environment or nurture, than of genetic or nature.
However one weakness is that some approaches, like the cognitive approach acknowledge the contribution of both aspects of environment and genetics, suggesting it may be that an interaction between the two factors that is more realistic. 
Regarding neural plasticity, One study of London Taxi drivers showed the region of their brains associated with spatial memory was bigger than in controls- this is not because they were born this way but because their hippocampi has responded to increased use (Maguire et al.2000)
Another study found that the region of the brain that controls finger movement increased in size in participants required to play piano finger exercise daily over the course of 5 days, and a similar effect occurred when participants merely imagined doing exercises. Studies with non-human animals have observed the way neurons shrink and grow in responses to changing environmental conditions (e.g. when exposed to light and dark) (Edwards and Cline, 1999)

In conclusion, it is more likely that behaviour can be explained as a mix of the two factors instead of just one factor. Alternatively behaviour can be explained by diathesis stress model is more likely biologically predisposed however a stress fully event is required to trigger the behaviour.