This paper studies the causal effect of birth spacing (i.e., the age difference between siblings) on personality traits. We use longitudinal data from a large British cohort which has been followed from birth until age 42. Following earlier studies, we employ miscarriages between the first and second child as an instrument for birth spacing. The results show that a larger age gap between siblings negatively affects personality traits of the youngest child in two-child households. This result sheds a first light on the causal effects of birth spacing on personality traits.