The current master thesis investigates potential causal explanations related to family structure behind what is now known as the Flynn effect. This refers to an increase in the average ability level in a given population over generations. Whereas several large-scale studies have established the relationship between a subject s ability level and this person s rank in the birth order as well as size of the sibship, changes in these effects could possibly have influenced the Flynn effect. Earlier born persons and those of smaller sibships in general obtain higher IQ scores than later borns and those of larger sibships. Hence, the assumption that changes in these effects related to family structure could have brought forth the increments of the Flynn effect was investigated. Further, the result of changes in the population ratios was also explored, whereas a shift from larger towards smaller families theoretically would yield higher IQ scores. This study is based on data obtained from two different sources. Data on ability scores stem from the psychological ability tests which the Norwegian Armed Forces administers as a part of the draft board assessment of Norwegian conscripts. These have been obtained from sesjonsdatabanken via Vernepliktsverket. This data material contains a General Ability score (made up three subtest scores) for the majority of Norwegian conscripts tested in the draft cohorts from 1969 through 2003. Additionally, data describing the family background of the subjects (rank in the birth order and sibship size) have been obtained from Statistics Norway, enabling an investigation of the relation between family structure and ability level. Analyses established changes in demography as an explanatory factor influencing the Flynn effect, as it was found to account for one fourth of the total IQ score gains in Norway in the relevant period. This signalizes the importance of investigating the developmental changes in population ratios to achieve a better understanding of the secular gains of the Flynn effect. No significant change could however be detected as a function of the birth order effect or the effects of sibship size relevant to the development of the Flynn effect. Whereas all sibships and all birth ranks increased with relative uniformity the influential source was assumed to operate at society level rather than at family level.