of best friends when conducting the genetic analyses by allowing the threshold to vary as a function of the number of shared friends [41]. The threshold is modelled as a simple linear function: ti = t + shared best friendsi ts, where t is the population baseline threshold (for individuals with 0 shared best friends), ts models the regression of the threshold on the number of shared best friends, and shared best friends i is the number of shared best friends of the individual i [41]. The same approach was used to control for possible age effects (see below).
