We extend the study of behavioural types in voluntary contribution games, adapting the elicitation method of Fischbacher et al. (2001) to a broader range of economic and strategic incentives. Our results in the standard VCM game align with previous findings in many respects; in particular, we identify one-quarter of participants as a distinctive group of “strong” conditional cooperators. We provide an explanation for the behaviour of this group by tracking their contribution strategies as the financial incentives of the game vary. We find that conditional cooperators follow a sophisticated rule, matching contributions only when doing so leads to an overall welfare improvement. This favours an account of conditional cooperation based on social norm compliance, rather than confusion, inequity aversion, or warm-glow giving. Keywords: public goods, conditional cooperation, sophistication, experiment.