5%). For the test of childhood maltreatment, the RD is the absolute increase in the probability of depression with each grade of maltreatment (e.g. an RD of 0.1 in the Dunedin study would mean that the probability of depression increases by 10% with probable as opposed to no maltreatment and with definite as opposed to probable maltreatment). For the test of the 5-HTTLPR-maltreatment interaction, RD is the increase in the probability of depression per each short allele and unit level of childhood maltreatment over and above the summation of risks of genotype and maltreatment. Tests of the same hypothesis in the two samples were combined with the Fisher’s trend method, taking into account the direction of effect (Fisher, 1932).
