Figure 6.1. The multifactorial threshold model. This model assumes that there are a number of different risk factors that contribute in an additive way to an unmeasured, continuously distributed variable called the liability. Other words for liability would be predisposition, vulnerability, and susceptibility. In practical sense terms, everyone has a certain degree of liability to a disorder; it is just that some people have more liability than others. Once, the liability reaches a certain value (the threshold). a disease process begins. Ultimately, the phenotype is measured as a dichotomy (affected versus not affected) even though the underlying causes form a continuous dimension of liability.