‘Social cognition’ refers to different ways in which we see the world. It is important when we communicate with other people. For example, we need to know how people feel, perhaps by recognizing the emotion in their faces. Research has suggested that drugs like cannabis, tobacco and alcohol can make us worse at social cognition. Using data from the Children of the 90s study, we have shown that cannabis and tobacco use in adolescence is linked to later problems with social cognition, as well as with worse mental health (in particular and depression, anxiety, antisocial behavior, and attention deficit hyperactivity disorder). However, people often use these drugs together (e.g., smoking cannabis and tobacco together in joints), which makes it difficult to know whether it is cannabis or tobacco that is important in this relationship. We are proposing to use a new technique, called Mendelian randomization, which will allow us understand this better. This technique uses genes linked to either cannabis or tobacco use. If either cannabis or tobacco use is indeed causing the problems with social cognition, this will provide important information for policy makers as they decide how to tackle the use of these drugs in young people.