For some time, researchers have been interested in determinants of long-term growth which are not purely economic in nature. The development of the concept of social capital by sociologists, politologists and economists is a visible effect of this interest. Any determinant of long-term growth is obviously important from the point of view of sustainability, particularly when its relation to other factors of sustainability, such as environmental protection, is taken into account. This article presents the results of research into spatial variation in social capital within Poland (according to region and size of home town). The authors consider social capital in relation to trust, trustworthiness and, in particular, cooperation, as well as taking into account attitudes regarding legal norms (law-abidingness). Such a wide approach enables us to eliminate the negative aspects of trust and concentrate on its positive forms, which can result in an economic rent. The aim of the article is to identify regions of Poland that are characterized by a high level of social capital from the point of view of individuals exhibiting a willingness to cooperate with others and observe the law and relate these results with the perspectives for long-term growth in Poland. The results of the research are based on a study carried out on a sample of 1540 students using experimental game theory alongside a questionnaire that investigated various aspects of cooperation, trust, trustworthiness and aversion to inequality. The results of the study indicate intuitively reasonable associations between the data from the questionnaire and behaviour in the experimental games suggesting that the declarations are (in statistical terms) meaningful. Analysis indicates that, in Poland, readiness to cooperate tends to be higher in rural areas. Although the study concentrates on the relation between social capital and economic development, it is also argued that high levels of social capital also favour other components of sustainable development.