  We investigate an agent-based model for the emergence of corruption in public
contracts. There are two types of agents: business people and public servants.
Both business people and public servants can adopt two strategies: corrupt or
honest behavior. Interactions between business people and public servants take
place through defined payoff rules. Either type of agent can switch between
corrupt or honest strategies by comparing their payoffs after interacting. We
measure the level of corruption in the system by the fractions of corrupt and
honest agents for asymptotic times. We study the effects of the group size of
the interacting agents, the dispersion with respect to the average salary of
the public servants, and a parameter representing the institutional control of
corruption. We characterize the fractions of honest and corrupt agents as
functions of these variables. We construct phase diagrams for the level of
corruption in the system in terms of these variables, where three collective
states can be distinguished: i) a phase where corruption dominates; ii) a phase
where corruption remains in less than $50\%$ of the agents; and iii) a phase
where corruption disappear. Our results indicate that a combination of large
group sizes of interacting servants and business people and small dispersion of
the salaries of public servants, contributes to the decrease of systemic
corruption in public contracts.
