Anita Thapar, Frances Rice, Dale Hay, Jacky Boivin, Kate Langley, Marianne van den Bree, Michael Rutter, Gordon HaroldWe thank the authors for their interest in our article (). They raise a question about gender effects and highlight “it would be informative to look at the results stratified by gender.” We agree that examination of gender effects, where feasible, is important. To meaningfully examine associations between smoking in pregnancy and attention-deficit/hyperactivity disorder (ADHD) among genetically related and unrelated mother-child pairs, further stratified by child gender and exposure to smoking in pregnancy, a substantially larger sample size would be required. However, to address the points raised by Obel et al. (), we provide the data stratified by gender, with the caveat that the sample size available becomes too small in the unrelated group to derive meaningful interpretations.