We investigate whether TV watching at ages 6–7 and 8–9 affects cognitive development measured by math and reading scores at ages 8–9, using a rich childhood longitudinal sample from NLSY79. Dynamic panel data models are estimated to handle the unobserved child-specific factor, endogeneity of TV watching, and dynamic nature of the causal relation. A special emphasis is placed on the last aspect, where TV watching affects cognitive development, which in turn affects future TV watching. When this feedback occurs, it is not straightforward to identify and estimate the TV effect. We develop a two-stage estimation method which can deal with the feedback feature; we also apply the ‘standard’ econometric panel data approaches. Overall, for math score at ages 8–9, we find that watching TV during ages 6–7 and 8–9 has a negative total effect, mostly due to a large negative effect of TV watching at the younger ages 6–7. For reading score, there is evidence that watching no more than 2 hours of TV per day has a positive effect, whereas the effect is negative outside this range. In both cases, however, the effect magnitudes are economically small. Copyright © 2010 John Wiley & Sons, Ltd.