A new brain imaging study indicates that individuals who behave honestly don't do so because they suppress egoistic tendencies to act in their self interest, but because they lack such tendencies.
Why would we be averse, or even indifferent, to cheating when we could benefit from it? Perhaps because our automatic responses have evolved in social environments where self-interested behavior in the short-term has not always lead to personal gains over the long-term. Gaining a reputation as a cheat would be a one-way ticket to ostracism. Having intuitions sensitive to equity and the needs of others would promote the formation and maintenance of cooperative relationships that would ultimately be of benefit to the individual.