It is commonly believed that birth order is an important determinant of success. However, previous studies in this area have failed to provide convincing evidence that birth order is related to test scores, education, or earnings. Using data from the National Longitudinal Survey of Youth–1979, we investigate the association between birth order and adolescent behaviors such as smoking, drinking, marijuana use, sexual activity, and crime. Our estimates show that middle borns and last borns are much more likely to use substances and be sexually active than their firstborn counterparts. These results provide the strongest evidence to date that birth order is related to measurable behaviors. (JEL I12, J12, J13)