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Describe and evaluate how genes may be involved in aggressive behaviour.

According to several research, aggression can be passed on within family and relatives through genes, such of these research involves twin and adoption studies. There are two types of twins, monozygotic and dizygotic, monozygotic or identical twins share the exact same genes while dizygotic (DZ) or non-identical twins share only 50% of their genes. In twin studies, researchers compare the degree of similarity for a particular trait (such as aggression) between sets of MZ twins to the similarity between sets of DZ twin. If both types of twin were placed into the same environment and the results show that MZ twin are more alike in terms of their aggressive behaviour, then this should be due to genes rather than environment.

Research showed that in humans, aggressive behaviour is more correlated on identical twins than in fraternal twins. McGuffin and Gottensman (1985) found a concordance rate of 87% for aggressive and anti-social behaviour for MZ twins compared with 72% of DZ twins. They have also found that the aggressive behaviour shown by the DZ twins relates to its family environment. Such studies support the importance of genetic factors in aggression, but also highlight the significance of environmental factors particularly among young people.

Another study is of Mason and Frick (1994) where they carried out an analysis of 12 twin studies (and three adoption studies) involving 3795 twin pairs concluded that approximately 50% of the difference between antisocial and non-antisocial behaviours could be attributed to gene factors, with large estimates of genetic influence found for more violent behaviour that for less violent behaviour. Since the study is a meta-analysis, there are no ethical issues how ever, it may be difficult to establish cause and effect since its a correlation study. The study used a good sample increasing it reliability but then may have some researcher bias as it may depend n the researcher to interpret what aggressive behaviour is.

Coccaro et al (1997) assessed the degree of genetic and environmental influences on aggression in male participants. In this study, he investigate aggressive behaviour rather than more general antisocial or criminal behaviour. Data or 182 MZ twin pairs and 118 DZ twin pairs were analysed. From the data, they estimated that genes account for more than 40% of the individual differences in aggression and environmental influences accounted for around 50%of individuals differences in physical aggression and about 70% of individual differences in verbal aggression. he study may had some researcher bias since if depend on the researcher to identify what an aggressive behaviour is and it also has gender bias since only males twins were involved in the study. It will also be difficult to establish cause and effect because the study was a correlation study.

A second way of studying genetic factors in aggressive behaviour is by studying children who have been brought up by adults who are not their biological parents. Thus, adoption studies can help to untangle the relative contribution of environment and heredity in aggression. WHen the results show a positive correlation between the adopted children and the aggressive behaviour of the biological parent, a genetic effect is implied. But if a positive correlation is found between the adopted twins' aggressive behaviour and the non-biological family then an environmental effect is implied.

This claims have been supported by several studies. Hutchings and Mednick (1975) carried out a study of over 14, 000 adoptions in Denmark found that a significantly number of adopted boys with criminal convictions had biological parents (particulary fathers) with criminal convictions providing evidence for a genetic effect. The study used a a good sample number however lacks population validity and have culture bias since it was carried out in Denmark, results may not apply to other cultures. Gender bias also occurs since the only participant used were boys.

Miles and Carey carried out a meta-analysis of 24 twin and adoption studies that had investigated the genetic basis of aggression. Most studies had relied on parental or participants' self reports of aggressive tendencies, although some involved observation of aggressive behaviour. The results suggested a strong genetic influence that could account for as much of 50% of the variance in aggression. Age differences were notably important, but at later ages the influence rearing environment decreased and the influence of genes increased. A later meta-analysis was carried out by Rhee and Waldman (2002) where they combined results of 51 twin and adoption studies and concluded that aggressive antisocial behaviour was largely a product of genetic contributions. However, as with the Miles and Carey study, several variables including age and assessment method for aggression, moderate the genetic influence on the aggression, suggesting that although genetic factors play a significant part in the development of aggressive behaviours, the influence of other factors affect their expression. Since both studies were carried out as meta-analysis, there were no ethical issues, although researcher bias may occur as it depends on how the researcher interpret aggressive behaviour. Demand characteristics may also occur since its an observation studies.

There have been criticisms on how the twin and adoption studies were carried out. Finding a large number of MZ twins who are rears is extremely rare and therefore, in a classic twin study, researchers must compare MZ and DZ twins in order to assess which type of twin pair is more similar on a given trait. Consequently, twin studies are not the perfect genetics versus environment experiment that we would like to imagine they are. Most studies claim that aggression are passed on to males (Hutchings and Mednick) and this claim has been criticised by Button et al. (2004), in a study of 258 twin pairs aged 11-18; Button found that both aggressive behaviour and non-aggressive anti-social behaviour are subject to significant. genetic behaviour however was higher in girls than boys. This results suggest a stronger genetic effect on aggression in females than in males.

There are also problems with interpretation of adoption studies. Adoption studies have number of methological problems which limit the conclusion that can be drawn from them. An example of this are countries such as New Zealand and the US where the children given up for adoption has shown higher display of anti-social behaviour. Thus, its difficult to say whether it is purely due to biological factors orfeelings of abandonment that lead to aggressive behaviour (Fergussion et al. 1995).

Another explanation of genes being involved with aggression is through candidate genes. Studies in this area typically examine whether one particular variant of a candidate gene occurs more often in people who display aggressive behaviour than in a comparison group. Imbalance of serotonin and dopamine are thought to increase aggressiveness. Associations have been found between genes associated with both serotonin and dopamine pathways. Farone et al. carried out a meta-analysis of studies of the gene for this receptor (DRD4) found a modest association between DRD4 and a tendency to attention deficit hyperactivity disorder or ADHD. Retz et. al. (2003) found an association between DRD3 variant and both impulsivity and ADHD-related symptoms in violent offenders.

Another gene factor is MAOA. MAOA also known as monoamine oxidise A is the gene responsible for producing protein has been associated as responsible for aggressive behaviour. MAOA regulates the metabolism or breakdown of serotonin, dopamine and noradrenaline in the brain. This has been supported by Brunner et al. where he carried out a study involving a Dutch family and found out that the males behaved more in a violent and aggressive manner. These men were found to have abnormally low levels of MAOA in their bodies resulting in extremely high levels of the three neurotransmitters which supported against the idea that a lowered levels of serotonin can increase aggressive behaviour. The study lacked population validity and is culturally biased as it was carried out on a Dutch family, therefore, they cannot be generalised for other cultures and countries. The study also concentrated in males, leaving female member of the family therefore is gender biased.

The explanation has been criticised by several studies. Most of the studies taken for the explanation had limitations such as difficulty of establishing cause and effect. The study also doesn't take note of environmental factors, and this claim has been supported by Capri et al. where they studied the relationship between aggressive behaviour and MAOA on 500 children and found that when children have been maltreated and had low levels of MAOA, they are likely to have an antisocial, aggressive behaviour as adults. However, the presence or absence of environmental factors cannot be identified by genetic factors. These findings may also have positive imlipactions of gender researches as they can be used for offender treatment and rehabilitation.