Using data we collected in rural Burkina Faso, we examine how children's cognitive abilities influence resource constrained households' decisions to invest in their education. We use a direct measure of child ability for all primary school-aged children, regardless of current school enrollment. We explicitly incorporate direct measures of the ability of each child's siblings (both absolute and relative measures) to show how sibling rivalry exerts an impact on the parent's decision of whether and how much to invest in their child’s education. We find children with one standard deviation higher own ability are 16 percent more likely to be currently enrolled, while having a higher ability sibling lowers current enrollment by 16 percent and having two higher ability siblings lowers enrollment by 30 percent. Results are robust to addressing the potential reverse causality of schooling influencing child ability measures and using alternative cognitive tests to measure ability.