Birth order and post-traumatic stress disorder

Objective: To compare the birth order of patients with post-traumatic stress disorder (PTSD) and adjustment disorder (AD) with population norms. Method: 83 PTSD patients and 104 AD control patients from a psychiatric trauma clinic were diagnosed according to DCR-10 guidelines. A family history was taken as to number of siblings, and their birth order. We compared the distribution of birth order for each patient group against birth order distributions expected by chance for the same years of birth using UK population-level birth order from the Office for National Statistics. Results: Psychiatric patients with PTSD were more likely to be from a large family, specifically to be the fifth child or later (OR 4.78, p<.001) and less likely to be the eldest child (OR .65, p<.001) than the general population in England and Wales. There were no differences for birth order between AD patients and the general population. Conclusion: People with PTSD are more likely to be the youngest children from large families than expected from a random sample of people born in the same years. This association with birth order was not found for another psychiatric diagnosis AD from the same clinic. We discuss possible psychosocial and biological causes, and implications for further research.