NEW YORK ( TheStreet) -- The horrific mass shooting at a public elementary school in Newton, Conn., in December thrust the thorny questions about runaway gun violence in America into the national spotlight. For investors, one of those questions is clear: what responsibility do we hold, if any, for the social ills caused by businesses in which we are stakeholders?The California State Teachers' Retirement System, known as CalSTRS, signaled its answer after the Newton shooting by voting to sell off its holdings in gun manufacturing companies. That appears to have prompted private equity firm Cerberus Capital Management to sell off its stake in the Freedom Group, a company that manufactures the Bushmaster semiautomatic rifle that authorities say was one of the murder weapons used in Newton. AAPL) labor practices socially-responsible? What about its tax avoidance schemes?