What church examination records can tell us about the inheritance of reading disability.

Long before the establishment of a general school system in Sweden (1842), a vast majority of the adult population had reached some reasonable level of reading literacy. The level of reading skill among the members of a household was assessed by the parish priest at annual catechetical examinations, and the results of these examinations were recorded in church registers. Eventually (in the 18th century) a 5-point grading scale was developed. In the present investigation, these unique records were used to study the transmission of low reading marks over successive generations in 17 families. For comparison, a set of 17 family trees originating from good readers was traced. The average scores of the descendants of poor readers were significantly lower than corresponding scores for descendants of good readers. However, the transmission patterns in the family trees of poor readers did not indicate any simple genetic mechanism. In another study, two dyslexic cases living today were traced backward to ancestors born around 1750. In one of the cases, a massive familial pattern of reading disability down to the eighth generation was observed, while most ancestors of the other case had average or above average reading skill as judged by the priests. The reasons for the absence of a simple hereditary pattern were critically discussed.