Until the beginning of the 20th century, it was a common educational practice in most Western countries to enforce right-handedness to spare naturally born left-handed children disadvantages in later life and especially in the labor market. Our study investigates whether this “childhood intervention policy” was successful in improving labor market and other economic outcomes of natural left-handers in Germany, where handedness conversion was officially part of the educational policy up to the 1960s and practiced even much longer. We distinguish between three groups, the natural right-handers, the natural left-handers, and the “converted” left-handers, i.e. natural left-handers who use the right hand for writing. Furthermore, we exploit variation in exposure to handedness conversion due to differences in educational policies over time and across federal states. We find that the earnings of natural left-handers in Germany do not differ significantly from those of right-handers. In contrast, converted left-handers suffer from substantial wages losses, even when controlling for a large number of socio-economic characteristics. We look at potential mechanisms, and find that some personality traits differ significantly between natural and converted left-handers and that the group of converted left-handers performs less well in an cognitive skill test. We conclude that handedness conversion can be a massive interference for individuals’ physical and psychological development.