A mobile ad hoc network, is an independent network of mobile devices connected by wireless links. Each device in a MANET can move freely in any direction, and will therefore change its links to other devices easily. Each must forward traffic of others, and therefore be called a router. The main challenge in building a MANET is in terms of security. In this paper we are presenting the mathematical model to detect selfish nodes using the probability density function. The proposed model works with existing routing protocol and the nodes that are suspected of having the selfishness are given a Selfishness test. This model formulates this problem with the help of prior probability and continuous Bayes’ theorem.