LAUSR

INTRODUCTION There is an argument about inbreeding as a risk factor in the development of major mental illnesses like bipolar disorder or schizophrenia. The genealogical trees of European royal dynasties represent an opportunity to study this relationship. Felipe V was the first Bourbon king to rule in Spain, who was thought to have possibly suffered a severe mental disorder. Felipe's son Fernando VI, also had a mental disease. Nevertheless Carlos III, Felipe V's son born from a different, unrelated mother, apparently showed no trace of such disorder. We hypothesize that consanguinity or inbreeding could explain the tendency in the acquiring of severe mental disorders. SUBJECTS AND METHODS  Biographies of three Spanish Bourbon kings (Felipe V, Fernando VI and Carlos III) were analysed in order to evaluate if they were likely to have suffered from some mental disorder, according to DSM-5 and ICD-11 criteria. An inbreeding coefficient was determined with the PedPro Program. RESULTS  Felipe V and Fernando VI could have been diagnosed with bipolar disorder, whereas Carlos III could have been diagnosed with a mild personality disorder. Both Felipe V and Fernando VI had a high inbreeding coefficient (0.091109 and 0.095023, respectively), while Carlos III had a lower one (0.038264). The inbreeding coefficient of siblings of couples of first grade cousins is 0.0625. CONCLUSIONS  A high inbreeding coefficient is a risk factor in developing bipolar disorder. Felipe V and Fernando VI suffered from bipolar disorder. The analysis of genetic trees by the inbreeding coefficient is a valuable tool in the study of heredability of bipolar disorder. Endogamy accumulates pathogenic alleles and increases risk to suffer it. These kings had a high consanguinity which was an important risk factor to the illness. High consanguinity is a risk factor to suffer bipolar disorder.