 By itself, therefore, the surface fact of a social gradient in health is compatible with quite different accounts of the underlying causal influences on individual health. Each distinct social variable might function as a “marker” for a different underlying causal factor, different social variables might function instead as alternative markers for the same underlying causal factor, or there may be some mixture of both. It is also possible that some social variables—e.g., education (see Cutler, Lleras-Muney, &amp; Vogl 2011)—function as a relatively direct causal factor.