This research argues that the rigidity of fertility decisions brings about over- and under-investment in education, thereby delaying human capital accumulation and economic growth. In underdeveloped stages, large family sizes prevent parents from revising their education plans upward even if their children are unexpectedly competent. In advanced stages, by contrast, having fewer children leaves a generous budget for education, which discourages downward revisions for children who are unexpectedly incompetent. The impact of these 'lock-in effects' depends in part on the degree of parental altruism.