Inbreeding Coefficient F and the Habsburg Dynasty Collapse

The inbreeding coefficient F measures how much of an individual's genome is identical by descent at both copies — essentially the probability of being homozygous at a random locus because both alleles trace back to the same ancestor. F is the cleanest single number for comparing inbreeding risk across relationship types. Key values: full siblings or parent-child matings produce offspring with F = 0.25 (catastrophic — territory of Charles II of Spain). Half-siblings produce F = 0.125, roughly twice the genetic load of first-cousin offspring. First cousins produce F = 0.0625, the historically most common form of legal consanguineous marriage. Second cousins fall to F = 0.0156, statistically indistinguishable from unrelated. Mapped to birth defect rates against a baseline of ~3% in unrelated populations: first cousins run 4-6% (about 1.7x baseline), half-siblings run 6-8% (about 2x), and full-sibling or parent-child offspring run 25-40% — catastrophic but observed historically. The textbook case of inbreeding depression is the Spanish Habsburg dynasty. Roughly 80% of Spanish Habsburg marriages between 1516 and 1700 were consanguineous, including uncle-niece and double-first-cousin pairings. The inbreeding coefficient compounded from F = 0.025 in Philip I to **F = 0.254 in Charles II** — equivalent to parent-child mating, accumulated over five to six generations of repeated cousin marriages. Charles II suffered mandibular prognathism (the "Habsburg jaw"), severe intellectual disability, epilepsy, and infertility, dying childless in 1700 and ending the Spanish Habsburg line. Across the dynasty, offspring survival dropped roughly 18% per generation as genetic load accumulated faster than selection could purge it.