Birth order in sex-offending and aggressive-offending men

The relation between birth order and number of sexual and violent convictions was investigated retrospectively in a sample of 113 men, to determine whether the established fraternal birth order effect in male homosexual preference and deviant sexual preference (e.g., for rape and pedophilia) may be extended to sexual behavior. Participants were mentally disordered offenders detained in a medium secure psychiatric unit in the United Kingdom and comprised 64 men with sexual convictions and 49 men with nonsexual violent convictions. Sibling data from psychiatric notes were used to calculate Berglin's birth order index for each participant and conviction data were obtained from the Home Office Offenders' Index. Fraternal birth order was significantly correlated with number of sexual convictions (p < or = .05). No association was found between fraternal or sororal birth order and violent convictions in either the sex offender or violent offender group. Results suggest that the fraternal birth order effect previously found for deviant sexual preference in sex offenders (K. Côté, C. M. Earls, & M. L. Lalumière, 2002; M. L. Lalumière, G. T. Harris, V. L. Quinsey, & M. E. Rice, 1998) also applies to sexually deviant behavior and is specific to sexual rather than general offending. Results are discussed in terms of the maternal immunosensitization hypothesis.