This paper provides experimental evidence to Crudeli's (2004) theoretical conjejcture, according to which the distribution of Social Capital is determined by the set of available economic opportunities. We find evidence that, after accounting for natural cooperation (the level of cooperation simply determined by incentives), excess cooperation\\ remains indeed influenced by the distribution of opportunities, becoming a more important determinant of cooperation the scarcer these opportunities are. On a general ground, when opportunities are fewer, people are more solidary to each other, more prone to reciprocate positive behaviours as well as more sensitive to betrayal. We also discuss our results on the background of the on-going debate on the measurability of Social Capital.