Economists and psychologists argue that individuals skew personal beliefs to accord with their own interests. To test for this presence of self-serving beliefs, 1,200 members of the Mormon Church were surveyed about tithing. A tithe is a voluntary contribution equal to 10% of income. Since respondents must decide privately what income items to tithe, how the income definition depends on an individual's religious and financial incentives was observed. Surprisingly little evidence was found that an individual's financial situation influences beliefs about what counts as income for the tithe. However, ambiguity increases the role for self-serving biases.