There is a male excess in many samples of patients with dyslexia and related conditions. Various suggestions have been made to account for this, including: (1) ascertainment bias, in that boys are more likely to be referred than girls; (2) a biological propensity of parents of dyslexic children to produce a high proportion of boys (compared with parents of controls); and (3) greater susceptibility among boys than girls to these conditions. These explanations are not mutually exclusive, and the first undoubtedly applies to some clinic samples. This paper offers equivocal evidence for the second explanation, and introduces new evidence for the third.