Unlike simple monogenic and some complex traits that are relatively static (e.g., stature in adults), liability to SUD is dynamic, likely characterized by a nonlinear developmental trajectory (Tarter and Vanyukov, 1994). As shown in Fig. 1, concrete organismic and environmental factors can be conceptualized as vectors projecting onto the liability axis, analogous to vectors of force in physics. These vectors determine the direction and position of the trajectory at any point in time, with the initial location of the individual phenotype on the liability axis at the moment of conception. Considered as a vector, even a small relevant functional genetic or early environmental difference, being at the origin of a causative chain as it is, gives an individual behavioral phenotype the initial push that may result in a large deviation in later development, accumulating momentum. At the relatively distal time when genetic relationships are usually evaluated, i.e., when a disorder or a behavioral deviation can be observed, the role of this early-acting factor may be as difficult to detect as it is to find a match that started a forest
