In a delay tolerant network (DTN) the nodes may behave autonomously deciding on their own whether to implement or not the rules of a routing algorithm. In this paper, the effects of node cooperation (or lack of it) are explored for three well-known routing algorithms proposed for DTNs with respect to the message delivery delay and the transmission overhead incurred until message delivery or the termination of the message spreading process. The results show that the sensitivity of the algorithms to the cooperation degree can be high, to the point of making them inferior to algorithms they typically outperform under a fully cooperative environment. Finally, it is demonstrated how a simple mechanism that incorporates the cooperation degree can help improve effectiveness.