This paper investigates whether celibate priesthood is fit. According to economic theory, the choice of celibacy can be rational for agents with a unique preference, viz., the consumption/production of “spiritual goods.” This paper provides a rational choice model that shows why, under some social constraints, agents select the precommitment device, viz., celibacy, in order to send a “credible” signal about the authenticity of the provided spiritual goods. This means that parishioners, under those social constraints, flock towards parishes headed by celibate priests. But such rational model would lead, once confronted with natural selection theory, to a puzzle. Namely, given that the preference for spiritual goods is based on a particular gene, and given the choice of celibacy, this gene cannot replicate itself. Thus, this gene is unfit if it entails celibacy, i.e., people with the need for spiritual goods would vanish from the population as long as celibacy is a necessary choice. This case illustrates how, at least apparently, utility maximization does not match fitness maximization.

But this need not be the case. The preference for spiritual goods, with the celibacy commitment, can be a fit trait.

To show how, this paper proposes the “expanded gene” hypothesis. The hypothesis supposes that the preference for spiritual goods is based on homozygote alleles. While the allele for the preference is recessive, the allele for non-preference is dominant. For a priest to assume his vocation, and for the parishioner to consume the spiritual goods, they must be carriers of the alleles of the recessive gene. In a model where there is no parish, there is low likelihood for the production of people that demand spiritual goods. In a model with a parish, where the priest is celibate, there is a greater chance for the production of people that demand spiritual goods. While priests cannot reproduce the gene for spirituality, the parish institutions allows the gene to have an “expanded” expression. The parish provides space for members who have the gene to meet and marry. In this manner, the parish secures a stable frequency of recessive alleles in the population, even when we have a depletion in the form of celibate priesthood.

The proposed expanded gene hypothesis is compared to other approaches—such the inclusive-fitness hypothesis and group selection theory—to solve similar puzzles. The proposed hypothesis can be extended to explain other puzzling social phenomena: i) monks, i.e., celibate priests without parishes; ii) heroes who sacrifice their lives for the benefit of a cause or saving life; and iii) philanthropists.