Objectivists agree that we should cooperate, but only if it's a rational trade. Thus, Objectivists wouldn't want to credit our social instincts. The problem, though, is that those instincts must be included in the objective reality we need to account for in deciding what to do. If we assume everyone is an antisocial egotist, we'll behave one way. But if we assume most people are inherently social and instinctively cooperative, we'll behave another way. It's a question of the empirical input we enter into our rational calculations.