A collective choice problem involves a set of agents and a set of feasible utility vectors. A solution is a (collective) choice function that assigns to each element in a family of admissible collective choice problems a unique feasible utility allocation. Our concern here is with solutions that are collectively rational, i.e. for which there exists an ordering of utility space such that the solution outcome to each choice problem is obtained as the maximal element of that ordering on the set of feasible utility allocations. An axiom due to Harsanyi, called bilateral stability, is used to obtain the following integrability result: Any solution satisfying Pareto optimality, continuity and bilateral stability can be represented by an additively separable Bergson-Samuelson social welfare function.