The author has examined 1437 crops of hair, removed from adult or adolescent males on admission to jail, at Cannanore, South India. There is a high positive correlation between weight of hair and rate of infestation with head-lice, men with less than 10 g. of hair being 14·6% infested, those with 10–19·9 g. 39·9%, and those with over 20 g. 56–59%.

There appears to be a negative correlation with age, but this seems to be due to the older men having less hair: if one standardizes hair weight there is no tendency for older men to be less infested than younger. In a similar way, the large differences in rate of infestation between different religious groups appear to be due to a great extent (but not entirely) to differences in hair weight.

No evidence was found that rate of infestation is different at different times of year. The rates in the rainy and dry seasons scarcely differ, in spite of the intensity of the rain, and of the fact that the rainless period lasts for six months. It is possible that there is a long-period, secular change in rate of infestation.