A UK-based study by Brown & Harris (2013) provided further evidence for the interaction between the 5-HTTLPR gene and early life stress. The research found that childhood maltreatment before the age of nine significantly increased the risk for chronic adult depression, but only among individuals with one or two short versions of the gene. For those with two long versions of the 5-HTTLPR gene, childhood maltreatment did not elevate their risk of developing chronic depression, highlighting the gene's role in moderating the effects of stress.