Genetic and environmental influence in human development

Meyers adopted the analogy of a tree in an attempt to explain human differences. According to him the trunk signifies the species, which then divides into branches. The branches represent our shared characteristics and beliefs. At the end of the branches are leaves, which Meyers claimed stood for the individual person, genetically and environmentally like no other.

Every human being is differentially sound. We all have our own genetic makeup with the exception of identical twins. In this case environmental factors will have caused differences in their development and behaviour in the same way they do for everyone else. When a child progresses through the early stages of its life, every aspect of his or her emotional, mental and physical abilities develops.

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In terms of the nature debate, evolutionary/biological/genetic psychologists believe our development relies heavily on our genetic make up. They believe we have no control over how we develop, but are merely the main actor in a play written and directed by our genes. These genes will hold information about our ancestors as well as the broader, evolutionary information that will allow us to ‘survive’ in the most effective way. Evidence has been provided to suggest that certain psychological problems or types of development are hereditary. Many studies show a high concordance rate of a certain behaviour or illness in families, especially the nuclear families, of the sufferer.

With regards to the nurture debate, psychologists who argue that environmental factors influence development have produced results supporting the view that human variation across cultures and over time show how differing norms or expectations guide a child’s development and that anything from peer pressure to mother-baby interaction can affect how well a child develops.

When a criminal psychologist tries to find the reason why a human being has taken away another’s life, they will examine the life events that took place during childhood. They will look at the effect of such experiences on the way the individual perceives themselves and the world around them and will ultimately try to find the reasons behind the crime. This is exactly the same as when a sufferer of schizophrenia has bouts of serious depression or violence, or a fear of spiders is analysed. It is strongly believed that the heart of these problems lie in events that occurred during childhood development and that to help the sufferer deal with the trauma will lead to some form of recovery or even an eradication of the symptoms.

As part of this theory, cross-cultural studies allow us to draw conclusions about the universal standards of our development. They can also help by leading us to appreciate both our cultural diversity and our human kinship. Studies have shown that regardless of our culture we share the same life cycle, learn to talk, walk, etc at around that same time in our lives. (Bernstein et al ’92). Rowe ’94/’95 found that cultural differences on the surface may have very similar foundations. A major criticism of cross-cultural studies is that of researcher bias. It is claimed to be very difficult for an American researcher to compare his/her own country to another and remain unbiased and objective.

A relatively new theory has adopted both the evidence for the nature and the nurture debate to investigate individual differences. This is the theory of behavioural genetics.

All three of these approaches attempt to find out how a child progresses through a broad range of developments including social, cognitive, intelligence and moral development.

To allow a deeper and more detailed explanation of each theory, it is easier to study them in context – within a particular area of development. Depression and aggression are suitable characteristics as much research has been conducted on them.

Depression is an affective mood disorder, which is usually characterised by feelings of sadness and withdrawal from those around us. The degree of this impairment can vary enormously. It can cover a range between mild depression, which is barely noticeable, to severe depression, which is debilitating in the extreme, often leaving a person unable to do basic tasks such as dressing and washing. At worst it can lead to suicide.

It is quite normal to feel depressed from time to time. Every individual knows what it is to feel ‘under the weather’ or ‘a bit down’. These feelings are a part of human life. They are usually short-lived and do not effect every-day functioning to any great extent. However, problems arise when the depression becomes clinical, in a psychological and medical sense.

There are two main causes of depression. The first is ‘reactive’ depression, which derives from an outside stimulus. The second is ‘endogenous’ depression, the causes of this lie internally and can arise independent of external events.

When an individual experiences either reactive or endogenous depression and remains in this state the depression is known as unipolar. When the depressive states associated with unipolar depression are combined with periods of mania and hyperactivity, the depression is referred to as bipolar (or manic) depression. A manic-depressive will express characteristics such as making reckless decisions, delusions, hallucinations and an increased sexual desire.

There is very little evidence for genetic causes of unipolar depression, even though the DSM-IV claims there is a high incidence of unipolar depression in the offspring of those with bipolar depression. However, it is quite the opposite for bipolar depression. The DSM-IV in 1994 conducted a family study and found that close relatives of depressed individuals had a morbidity rate of 4-24% compared to the 1-2% of the general population. A major criticism of studies involving families is that they share the same environment as well as genes.

Twin studies were adopted in an attempt to extract the environmental factors that may come into play. Price (’68) looked at seven twin studies and found a much higher concordance for depression in identical twins (MZ) as opposed to non-identical twins (DZ). The most interesting find of this study was that the concordance rate for MZ twins reared separately was only 1% different to the rate in MZ twins reared together. This ruled out environmental factors as well. To further reduce the environmental factors, adoption studies also provide strong genetic evidence. Cadoret (’78) studies 126 adopted children, eight of whom were born to a parent with manic depression and adopted by a non-depressive family. Three out of the eight developed a major depressive disorder compared to only eight in the remaining one hundred and eighteen children.

Perhaps the strongest evidence for genetic causes of manic-depression comes from studies on mutations on chromosomes. These studies try to identify DNA markers. Egeland et al (’87) studied the Amish community in Pennsylvania. It is a small, inbred community with a high incidence rate of depression. The study indicates that bipolar depression is related to two markers. The first is the insulin gene, and the second is the cellular oncogene Ha-ras-1. Unfortunately, replication studies including work by Berrettini et al (’90) have not supported this finding. Work on inbred communities and DNA is a new area of study within depression, but much more work will have to be conducted on it before any significance can be assessed or verified.

Although the evidence for genetic causes of depression is strong, no studies have produced results showing a 100% concordance rate. As a result of this, it has been assumed by many psychologists that genetic factors might be predisposing ones and that there may be additional precipitating causes.

In terms of unipolar depression, environmental causes provide the most plausible accounts. The psychodynamic view relates depression in adulthood to relationships in early life. Hostile feelings towards parents, for example, are directed inwards in the form of self-hatred. These can arise from many factors including lack of love or maternal deprivation. The famous conclusions drawn by John Bowlby through his examinations on attachment studies with animals, suggested that separation/loss of mother during infancy could result in severe depression in adulthood. Evidence for this belief came in the form of Hinde (’77) and his work on Rhesus monkeys. However Paykel (’81) reviewed fourteen animal experiments and concluded that seven provided evidence for Bowlby’s claim and seven did not. It should be mentioned that it is perhaps unwise to attribute results found in animal studies to human behaviour, as we are a completely different species. Studies of this nature may be more difficult to conduct in the new century due to an increase in the need of ethical approval for all experiments.

Major life events are also thought of as being contributors to reactive depression. This theory has been greatly supported to the point where it is now incorporated in the DSM diagnostic criteria. However, it does not explain why many patients do not report critical life events at the onset of their depression. It has been suggested as a solution to this problem that life events may interact with other vulnerability factors. The Diathesis-stress model has provided this. Lewisohn put the behavioural theory of social reinforcement forward in 1974. This suggested that depression arose from a reduction in positive reinforcement. In addition, the depression will increase the likelihood of getting sympathy from others. A criticism of this is that depression often carries on after the sympathy has waned.

One of the most notable behavioural explanations for reactive depression is the theory of learned helplessness. Seligman produced this theory in 1974 whilst studying Pavlov’s work on fear conditioning in dogs. He found that when animals were placed in places they could not escape from they made no effort to escape when placed in a situation they could escape from. According to Seligman, the onset of depression occurred as a result of a human being feeling that they cannot escape a situation. Seligman and Maier (’76) replicated these studies with human beings and found the same results, however Wortman and Brehm (’75) found that humans still tried to escape whether or not they had experienced being put in inescapable situations.

In conclusion, much research has been carried out on the genetic and environmental causes of depression. A genetic tendency for unipolar depression is not supported by twin and adoption studies, but there is strong evidence for a genetic component in manic-depression. Social factors appear to provide the most plausible accounts for unipolar depression.

Aggression and violence affect society in a variety of ways. It is the general consensus that our society is becoming increasingly more violent. However, it is shocking to realise the high level of violence that is committed by teenagers or even younger children. It is becoming almost common to pick up the local newspaper and read about a child, or group of children, who has committed a horrifying act on another human being. With each horrendous story the reader becomes desensitised to the violence in our world.

Many researchers believe that aggression is an innate behaviour and that its foundations lie within our genetic makeup. Biological theorists are examples of such researchers. Maxon (’98) studied aggression in mice and claimed that their genes affect one or more type. Lucki (’98) claimed that serotonin, a neurotransmitter in the brain, might affect social behaviours involving aggression and is a successful treatment for impulse aggression. According to the biological theories, aggression is caused by the genetic makeup of the individual and therefore the most successful treatment would be through the use of genetic engineering – eradicating the problem. The main strength of the biological/genetic theory of aggression is in the work on serotonin. Studies have indicated that the choice of treatment is obvious and has been shown to effectively reduce aggression. Despite this, there are a number of weaknesses to the theory. In genetical theories causation is not well established. There is a question concerning cause and effect. Is the gene causing the aggression or does the aggression modify the genetic structure to produce this new gene?

The psychoanalytical theory believes that aggression is innate. The major figurehead of this approach was Sigmund Freud. His work was conducted around the time of 1920. He believed that human behaviour is motivated by sexual and aggressive drives or instincts. He believed that aggression was innate and a personality characteristic. Psychoanalysists believe that aggression is an innate reaction, one that will always exist as a part of human nature, however they believe it should be controllable by the individual by the time they reach adulthood. Psychoanalysis is their preferred therapy. They try to encourage small forms of aggression but dissuade more disruptive forms. The psychoanalytical theory has strength in the fact that it is able to explain all forms of aggression. It lacks, however, supporting empirical evidence. Most of the theory is based on attestable and unobservable concepts. It is due to these constraints that the theory has hardly changed over the past ten years.

When researching the nature/nurture debate of aggression, it becomes apparent almost straight away how much more emphasis there is in support of the nurture view as opposed to the nature. There are two main theories concerning the environmental causes of aggression, the first of which is the behaviourist theory. In the 60’s behaviourism was the major driving force within the area of psychology. The world famous ‘Pavlov’s Dog’s’ experiment produced results suggesting that living organisms could be conditioned to behave in certain ways. That there behaviour could be manipulated through the use of punishment or reward. Hergerhahn (’97) claimed “it was the general consensus that most social behaviours were learned through reward and punishment and the association of these principles to other antecedent behaviours.” The behaviourist theory has a weakness in the fact that it would be na�ve to assume that all cases of aggression are learned via a stimulus-response manner. Other environmental causes of aggression have been produced that claim aggression can be learnt through imitation and modelling and this has forced many behaviourist theorists to take a more social cognitive/learning approach to the development of aggression. The theory that I refer to is that of the Social Learning Theory. This theory has built on the behaviourist’s beliefs and added the use of imitation and modelling. The theory explains the development of the aggression in children, including gender differences.

In their Bobo Doll experiment, Bandura, Ross and Ross (’61) compared the aggression in children who had witnessed an aggressive act and compared it to the level of aggression in other children, who hadn’t seen a violent act. They found that aggression was more frequent and severe in children who had seen violence than those who had not. They demonstrated that imitation might be responsible for some of the acquisition of some of the behaviours not explained by previous models. These results suggest that to avoid having a violent child you would need to ‘censor’ their world.

What they watch on television is particularly important. If children imitate violent acts then allowing them to watch adult films is unwise. Since this finding many correlational studies have been conducted on viewed aggression and aggressive behaviour. Paik and Comstock (’94) looked at 217 studies on this correlation and found a highly significant, positive correlation between TV violence and aggression. In keeping with the topic of media, Wiegman and van Schie (’98) examined correlation between the amount of time children spent playing video games and the amount of aggressive behaviour they displayed. The results of this study showed that the use of video games had no effect on the child aggression. This finding goes against the social learning theory.

Edwards et al (’94) studied the effects of conflicts between mother and son on the child’s aggression. They found that children who had longer and more intense conflict with their mothers were more aggressive. Another study in support of the social learning theory is that conducted by Reidy in 1997. He studied abused and neglected children and found that they were more violent than the average child.

A major strength for the Social Learning Theory, as mentioned before, is in the larger amount of empirically supportive evidence. Being so grounded in scientific research has allowed it to become a theory that is open to change and it has been elaborately developed over the past four decades.

There are no members of society that are free from the repercussions of aggression and violence and for many years psychologists have attempted to identify the causes behind it. It is clear that research done on aggression from the social learning approach have helped us to understand the nature of aggression. In knowing this, societies can take important steps to treat and prevent aggression so that we can live in a less violent world. Already, there have been changes that implement the results of these studies.

It can be concluded from the different theories and supporting evidence that many factors affect the development of an individual. According to the nature debate, 50% of our development is dependant on our genes. Genes predispose our shared humanity and all of our individual differences. It is also true, however, that our development can be affected by influential life-events and our social relationships. It would appear that both nature and nurture factors affect our development. In terms of the aggression example, our genes may predispose that males will be more aggressive than females but our culture may enhance this gender difference. Just because there is evidence to support both genetic and environmental factors, it does not mean that we can rely too heavily on predictions based on these assumptions. We are open systems, we can sometimes defy peer pressure and we don’t always go through major changes due to life-events. Philosopher Jean-Paul Sartre described excusing our failings by blaming our nature or nurture is ‘bad faith’. To sum up, the following quote by Meyers concludes:

“We are both the creatures and creators of the world, the products of nature and nurture combined.”