In a study comparing IQ scores of boys and girls over time, what is a primary threat to internal validity?

Maturation refers to the natural changes that occur within subjects over time, which can influence the outcomes of a study. In the context of a study comparing IQ scores of boys and girls, maturation might involve cognitive development that happens independently of the study's intervention or conditions. As children grow, their cognitive abilities evolve, which could lead to differences in IQ scores that are not necessarily a result of any specific treatment or comparison being made in the study. This change can confound the results, making it unclear whether observed differences in IQ scores are due to gender or merely reflect the natural progression of cognitive development.

In contrast, selection bias relates to how participants are selected for the study, which isn't directly related to changes over time in the same individuals. The use of different testing instruments could be problematic but would primarily impact the accuracy and comparability of results, rather than internal validity directly. Lastly, sample size, while important for statistical power and generalizability, does not directly threaten the internal validity of how changes in IQ are measured over the time period if sample size is managed appropriately across groups. Thus, maturation stands out as a critical threat to internal validity in a longitudinal comparison of IQ scores.