Suicidal behavior runs in families and is prevalent in adolescence. Case-control and family-based studies in this age group failed to find a genetic association that survived replications. Gene environment approach gave new hope for possible associations especially with the short allele of the serotonin transporter promoter polymorphism (5-HTTLPR). However, a recent meta-analysis raised doubts about the consistency of these findings. Some new structural and functional imaging data may shade light on the age-related and gender-related development of the brain. This review suggests a new approach to gene by environment and timing interaction to understand the interplay that leads to suicidality in adolescents and young adults.

Twins can provide unique opportunities to study causal influences on variation in human behaviors, development, and diseases. During the past 10 years, the number of twin registries has increased rapidly across the globe and we thought it timely to bring these to the attention of our readership. In this special issue, we invited papers on twin registries and cohorts from 28 countries representing five continents. Subjects covered include how to establish and maintain twin registries, accurately assess zygosity, collect biospecimens, and other important issues related to twin studies. This special issue shows that over 1.5 million twins and their families are participating in twin studies worldwide. Research interests will be highlighted, with the aim of fostering collaborative research.

The finding of a significant gene by environment interaction effect on depression of the serotonin transporter length polymorphism (5-HTTLPR) and the Number of experienced Life Events (NLE) was not replicated in two large meta-analyses (Munafo et al., 2009; Risch et al., 2009). These meta-analyses have been criticized on the grounds that large studies that get most weight in meta-analyses have the poorest measurement quality of life events and, as a consequence, do not find an effect. Another issue is the time frame across which the NLE are measured. Proximal life events appear to be better predictors of depression than more distal events. We present the results of analyses of the 5-HTTLPR × NLE effect on anxious depression and neuroticism scores in a sample of 1,155 twins and their parents and siblings from 438 families. The interaction effect was tested separately for NLE experienced across the life span and NLE experienced in the past year. There was a significant main effect of NLE on anxious depression and neuroticism, especially when these were experienced in the past year. No interaction with 5-HTTLPR was found for NLE either experienced across the life span or across the past year. Our results support the two recent meta-analyses. Given recent insights from genome wide association studies, it seems more useful to focus on the joint effect of several genes, that are, for example, part of the same biological pathway, in interaction with the environment, than on one candidate gene.