Blood is thicker than water

“Blood is thicker than water” names the expectation that family bonds explain acts of care. In this discussion, the phrase is first a plausible account of kin-directed altruism, then a limited explanation: vampire bats share blood with unrelated roostmates, offering a case in which cooperation may rest on repeated association rather than genetic kinship.

Kinship as a genetic measure

The argument gives kinship a precise genetic meaning. A parent and child are related by one half, as are siblings on average, though the genes shared by a particular pair of brothers can vary with the chance distribution of genes. The relatedness rule is offered as a practical way to compare relatives: identify their most recent common ancestors and count the generational steps from one individual up to an ancestor and back down to the other. It works for simple cases, but not when incestuous mating occurs, and it also breaks down in certain insects.

This calculation explains why the phrase can seem to fit altruism within families. A gene that prompted an individual to die saving five cousins would not, on average, be preserved by that sacrifice; saving more than two siblings, children, or parents could be enough, as could saving more distant relatives in greater numbers. The point is not that an animal consciously calculates family ties, but that a heritable disposition can spread when the relatives benefiting from it carry enough copies of the same genes.

Parental care is therefore treated as one form of kin altruism, not as a genetically unique bond. An adult is related to its own child and to an orphaned younger sibling by the same proportion, one half; in principle, a gene encouraging care for a younger sibling could spread as readily as one encouraging parental care. The account distinguishes that genetic equivalence from what is commonly observed: sibling care is much less frequent than parental care, and other factors, discussed later in the book, complicate the comparison.

The cultural force of family ties

Human kinship is not recognized through genes alone. People learn who their relatives are through what they are told, the names they use, formal marriages, written records, and memory. Anthropologists studying kinship often attend to these subjective and cultural understandings rather than to genetic relatedness itself. Ancestor worship, family obligations, and loyalties give kinship a prominent place in social life; blood-feuds and inter-clan warfare are presented as readily interpretable through Hamilton’s genetic theory.

The discussion also separates the social force of a taboo from its proposed genetic explanation. Incest taboos show how strongly people attend to kinship, but their genetic advantage is described not as altruism: the proposed explanation is that inbreeding can bring harmful recessive genes to expression. The text notes that many anthropologists do not favor this account, keeping the difference between a cultural rule and an evolutionary interpretation in view.

Animals, by contrast, need not identify relatives by conscious knowledge. The relevant question is what simple inherited rule could, under ordinary conditions, produce behavior that tends to benefit close kin. A tendency to favor individuals who physically resemble oneself could sometimes do so, but only statistically: the rule could misfire if circumstances changed, such as when a species began living in much larger groups. The same possibility is used to suggest that racial prejudice might be an irrational generalization of a kin-oriented tendency to favor similar-looking individuals. The expression thus points to a real pattern of family loyalty without guaranteeing that every apparent kin bond tracks genetic relationship.

When the expression fails to explain sharing

The vampire-bat example gives the expression a direct test. Bats that find food at night may return to the roost with a surplus, while others return empty; on a later night, their fortunes may reverse. G. S. Wilkinson observed bats donating blood by regurgitation. Of 110 witnessed regurgitations, 77 could readily be understood as mothers feeding their young, and many other cases involved genetic relatives. Those instances fit the family-centered explanation captured by “Blood is thicker than water.”

Some cases did not. Unrelated bats also shared blood, and the recipients tended to be frequent roostmates with whom donors had repeated opportunities to interact. This pattern matters because repeated interaction can support reciprocal cooperation: a bat that helps another may later benefit when circumstances reverse. The expression remains relevant to the many donations among relatives, but it cannot by itself account for sharing between unrelated companions.

The chapter’s closing contrast is between the familiar vampire myth and the observed social pattern. Gothic stories cast vampires as threats, while the image of nature as relentlessly competitive reinforces that fear. The author argues instead that particular claims about organisms must be tested against evidence; evolutionary theory offers principles, not detailed predictions for every species. The bats’ blood-sharing then supplies a different moral possibility: their bonds can rise above kinship, and the final image of loyal blood-brotherhood turns the expression into a question of scope. Blood may be thick, but recurring cooperation can bind individuals who are not relatives.