 L. A. Shapiro and E. Sober (2007) have developed an alternative line of argument that they hold to apply just as well to functional properties and properties that resist functional analysis. The distinction between common causation and realization plays a key role. Common causation, they note, can lead to an empirically based denial that one co-effect causes another. For example, tail length in mice offspring is caused by their parents’ genes, not by their parents’ tail length — and this can be empirically shown by cutting parental tails (which, of course, leaves their genotype constant) and observing no effect on tail length of offspring.