Behaviour in public good experiments is usually attributed partly to rational self-interest and partly to social norms and preferences. This paper examines if sensitivity to social desirability affects public good contribution and in what way. A pre-experimental measure of social desirability (SDS17) was used to match partners in a two-person public good game. Half the participants received experimenter approval based upon their investment. Contrary to predictions, the highest public good investment was by low social desirability participants in the approval condition. Social desirability was not positively related to pro-social behaviour. We consider its relation to experimental and social conformity.