The trajectory group approach posits that there are several distinct pathways to alcohol use. But for this to be true, most of the variation in alcohol use would have to be explained by a small number of powerful categorical influences (e.g., having a particular risk allele or experiencing a particular event). These influences would need to be common enough to form sizable groups with a homogenous and predictable developmental course. In this case, numerous small influences would still have some effect on alcohol use, much as they do on intelligence in people with and without Down syndrome, but most people would follow a course of use within the relatively narrow constraints of their trajectory.
