The article states that the covariates included child’s sex, child’s age, number of siblings, parity, birth weight, child breast fed for at least 6 weeks, child’s ethnic group, child being raised by lone parent, paternal age, child being raised in workless household, family income, mother’s educational attainment, and mother’s social class both the number of siblings and parity. Parity is measured by the mother’s parity at the birth of the relevant child, which, in almost all cases, is equivalent to birth order. In addition number of siblings was included as a covariate. We agree that higher birth order may lead to increased maternal experience and greater stimulation from older siblings, which will both potentially positively impact on the outcome measures. However we are confident that our covariates control for these possible effects as thoroughly as possible. Thus we are confident that any significant results are associated with maternal age and not the child’s birth order.

Competing interests: None declared