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The figure shows the relationship between depression and experience of stressful events and between depression and maltreatment in childhood, according to each of three 5-HTT genotypes: ss, sl, and ll. The data is presented as vertical bar charts. In part (a) the horizontal axis is labelled number of stressful events experienced, and two number categories are labelled: 3 or more than 3 stressful life events, and zero to 2 stressful life events. The vertical axis is labelled percentage depressed at 26 years and is marked from zero to 45 at intervals of 5 years. The results are very similar for all three genotypes for the zero to 2 stressful life events group, at 14 per cent depressed individuals. In the 3 or more than 3 stressful life events group, those with the ss genotype had the highest level of depression (40 per cent), sl individuals had an intermediate level (27 per cent), and ll individuals the lowest level (22 per cent). In part (b) the horizontal axis is labelled maltreatment in childhood, and two categories are labelled: probable plus severe maltreatment, and no maltreatment. The vertical axis is labelled percentage of 18–26-year-olds with depression and is marked from zero to 70 at intervals of 10 per cent. The results are very similar for all three genotypes for the no maltreatment group, at 30 per cent depressed individuals. In the probable plus severe maltreatment group, those with the ss genotype had the highest level of depression (58 per cent), sl individuals had an intermediate level (42 per cent), and ll individuals the lowest level (30 per cent).

 6.1  The serotonin transporter gene and vulnerability to stressful life events