Consanguineous marriages are strongly favoured in the state of Karnataka. Of 65,492 marriages studied 33.07% were consanguineous, equivalent to a coefficient of inbreeding (F) of 0.0298. The twinning rate was low, 6.9 per thousand, whereas the secondary sex ratio, 0.5221, was higher than in comparable major human populations. Consanguinity exerted no significant effect on either parameter. The results also indicate that consanguinity is not associated with excess antenatal losses and suggest the possibility of enhanced selection against mutations at X chromosome loci.