Loneliness is linked to a number of physical and mental-health problems including anxiety, schizophrenia, depression, poor sleep, a weaker immune system and cardiovascular disease. During adolescence loneliness is thought to peak early on and decline through middle and late adolescence but there has been little long-term research into this. Studies of twins have shown that there is a significant genetic effect on loneliness but no-one knows which genes are involved. Researchers from Radboud University in the Netherlands looked into both these issues in a study of 306adolescents. Loneliness was indeed found to reach a peak in early adolescence and decline thereafter. But, people with a certain variation in the 5-HTTLPR gene, which is responsible for transporting serotonin within the brain, were more likely to remain lonely throughout adolescence. For those children with the variation in the 5-HTTLPR gene receiving little support from mothers also increased the risk of loneliness.
van Roekel, Eeske ... [et al] - Loneliness in adolescence: gene x environment interactions involving the serotonin transporter gene Journal of Child Psychology and Psychiatry July 2010, 51(7), 747-754