 McShea (1994) gave one widely-cited formulation of the distinction between passive and driven trends, and also suggested several approaches for determining, empirically, whether a trend is passive or driven. One such approach is the stable minimum test: track, say, the body size of the smallest members of the clade. If that increases steadily over time, it is a signal that the trend is driven. Paleobiologists now routinely try to determine whether trends are passive or driven, though this work can sometimes encounter underdetermination issues (Turner 2009). Body size has received a lot of empirical attention, in part because it is easy to estimate from fragmentary fossil remains, and partly for the simple reason that it is a trait that every animal has.