 Schweikard and Schmid (see entry on  collective intentionality)  find more nuanced approaches in Scheler 1913 and Walther 1923. Scheler identifies various kinds of social entities, some of which involve people acting in solidarity, and Walther proposes an account of structured attitudes in terms of empathy. In the recent literature, Gilbert (1989, 1990) introduces new reasons for denying summative approaches: members of social groups are subject to norms that do not apply merely to individuals who have matching attitudes. If a couple is walking together to the store, one person violates a norm if she suddenly takes off in a different direction without excusing herself. But this would not violate any norms if collective intentions were merely summative. Gilbert argues that social groups, as opposed to mere collections of individuals, are formed by the members making joint commitments. She describes the formation of such commitments, and analyzes group attitudes in terms of them.