Caring for parents: an evolutionary rationale

The evolutionary roots of human moral behavior are a key precondition to understand human nature. Here we investigate whether a biological version of Fifth Commandment (\"Honor your father and your mother, that your days may be long\"), respected in different variants across cultures, can spread through Darwinian competition. We show by a novel demographic model that a corresponding Fifth Rule (\"During your reproductive period, give away from your resources to your post-fertile parents\") will spread even if the cost of support to post-fertile grandmothers considerably decreases the demographic parameters of fertile parents but radically increases the survival rate of grandchildren. Teaching vital cultural content is likely to have been critical for the value of grandparental service. Selection on such behavior may have produced an innate moral tendency to honor parents even in situations, such as experienced today, when the quantitative conditions would not necessarily favor the maintenance of this trait.