This copy is for your personal, non-commercial use only. To order presentation-ready copies for distribution to your colleagues, clients or customers visit http://www.djreprints.com. Affiliated Managers Group, the big holding company for asset managers, has agreed to buy Parnassus Investments, the socially responsible investment firm, for $600 million, according to a person knowledgeable about the transaction. The move demonstrates the popularity of environmental, social, and governance, or ESG, investing.