There are another two types of depression, but this fit into the original definition of depression. There is reactive depression, which is cause when someone experiences something stressful, such as the death of a loved one or extreme stress at work, which then triggers the depression. Endogenous depression is when there is a hormonal imbalance in the brain.Family studies, Gershon (1990)In 1990, a psychologist names Gershon presented the findings he had found after carrying out numerous studies on families, “in which depression was assessed in the first-degree relatives of patients with depression.” The rate were two to three times the rates of the general population, to develop both depression and bipolar disorder. Egeland et al (1987), claimed to identify that genes were a large part, while carrying out a study in a small religious community in Pennsylvania, which had low levels of depression. One family that was studied had a high level of bipolar disorder, and 81 members had manic depression. He suggested that the chromosome 11 was different in those with bipolar disorder or depression to those that did not suffer from a mental disorder. However, this research has not been replicated and it is not clear how Egeland et al cam to found exactly how the chromosomes are different. Even though it has been proved that genes play a part in whether someone develops depression it is not clear whether it is the chromosome 11 or not. However, these genes are the “neighbours” of the genes that produce monoamines, which is a biochemical linked to depression. The Department of Psychiatry at Columbia University, New York, decided to carry out a study on major depressive disorder and create a longitudinal study. This was in order “to examine the familial aggregation of psychiatric disorders and functioning in grandchildren by their parents’ and grandparents’ depression status”. This study observed and interviewed three generations with the family, and had one hundred and sixty one participants. The first generation (the grandparents) were interviewed four time during the entire period, which lasted 20 years. The results of this study showed clear links between the three generations of the family in relation to psychiatric disorders. It was found that 52% of the “grandchildren (mean age, 12 years)” were already suffering from a psychiatric disorder. Families that had grandparents with depression had an increased risk of any disorder, such as anxiety or depression. However, if the parents had depression but the grandparents did have depression, then there wasn’t a significant effect of the grandchildren in relation to depression. The mental health of the parents however, did have an impact on the lives of the grandchildren in terms of functioning. This study shows the role that genes play in the development of depression. However, it is not known whether there were variables out of control of the psychologist, as things such as stress can affect the mental health of a person. However, this is less likely when it comes to the children as their mean age is 12, and it is not often that a 12 year old suffers from enough stress in order for them to develop depression. Although, is their parents are depressed it would cause some stress for the child and they could for a mild depression or a disorder such as anxiety, which is common in children whose parents suffer from depression. This has also been supported by studies such as the adoption study, carried out by Wender et al (1986), in which he found that the “biological relatives of adopted sufferers from major depression were about eight times more likely than adoptive relatives to have had major depression themselves” and also found that if the adopted child later developed depression, the biological parents were eight times more likely than the adopted parents to have had depression. This type of research has gained reliability as it has been recreated many times with the same kind of results each time.Even though these studies support the idea that genetics are influential of the development of depression. The most supportive is studies that are based on those that concentrate on monozygotic and dizygotic twins. Monozygotic twins are identical twins, meaning that they developed from one zygote into two embryos. Dizygotic twins, also known as fraternal twins, are developed from two different eggs and two different sperm cells. By reviewing a number of studies for depression, Allen (1976) found that “the mean concordance rate was 40% for monozygotic twin”, which compared to the dizygotic at only 11%. Whereas for bipolar disorder it was 72% for monozygotic twins and 14% for dizygotic. Another study that was carried out by Bertlesen, Harvald, and Hauge (1977), “found a concordance rate for major depression of 59% for monozygotic twins” and “30% for dizygotic twins”. Which can be compared to the percentages for bipolar disorder in twins, which 80% for monozygotic and 16% for dizygotic. These studies and discoveries support the influence of genetics on the development of depression and bipolar disorder. However, it is not clear whether the environment that the twins are surrounded by has an influence on the development of the mental disorder. There has been studies on the behaviour of twins that have been separated their entire lives, but no significant ones on the development of depression for separated twins. By carry out a study that does this it would give more of an insight into the influence of the environment surrounding a person in relation to mental health. By study the environment, psychologists can see the relationship that an environment has with mental health and make it better for people who suffer from depression and can’t change their environment.