We investigate the impact of investment on growth in a sample of 85 developed and developing countries over 1984–2009, conditioning the marginal effect of investment on institutional quality. The panel structure of our dataset allows controlling for unobserved heterogeneity and dealing with the risk of endogeneity bias. We find that investment increases growth more in countries with high institutional quality than in countries with defective institutions. The results are robust to estimating the model separately for developed and developing countries, for each continent, and over two sub‐periods. A jackknife experiment shows that they do not depend on any single country. The results are essentially driven by the quartiles of countries with the lowest and the highest institutional quality, and by the Government instability, Corruption, and Rule of law sub‐components of the ICRG index.