Oh, brother

Would you rather be the heir or the spare? Tomorrow, the Duke of Sussex publishes his memoir: Spare. The title refers to the traditional practice of primogeniture, where the eldest son inherits everything from his parents. The younger brother is backup, insurance, a "reserve king".

This fraternalBrotherly. relationship can bubble with white-hot emotion. Jealousy, resentment, rage. For the princes, it has boiled over into a public media war. But throughout history, sibling rivalry has led to blood, conflict and the fall of empires.

The first murder. In Christian, Jewish and Islamic traditions, Adam and Eve's eldest son Cain grew jealous of his younger brother Abel. God accepted Abel's lamb sacrifice but refused Cain's gift of wheat. In a fury, Cain killed Abel.

Scholars argue this fratricideThe killing of one's brother or sister. may reflect tensions in the Bronze AgeThe time between 2,000BC to 700BC when people used bronze to make items rather than stones. between nomad herders and settled farmers.

Death in Rome. The origin story of the world's first superpower has a dark twist. According to tradition, twin brothers Romulus and Remus fought over where to build the eternal city. When Remus ignored an omenAn event seen as a sign that something good or bad is about to happen. from the gods, his brother's supporters killed him.

Romans were proud of this murderous myth. It stressed the importance of unity and obeying the gods.

Divided spoils. According to one medieval monk¹, William the Conqueror's three sons fell out over a prank. William Rufus and Henry dumped a chamber pot over Robert's head. He never forgave them, or his father for not punishing his brothers.

In 1087, William I died. He split his kingdom between them, setting the scene for civil war. William Rufus was killed in a suspicious hunting accident.² Henry threw Robert in prison and ruled for 35 years until he ate a delicious plate of poisoned eels.

Brothers in arms. The Wars of the RosesA series of civil wars that took place between between 1455 and 1485. Two competing royal houses, the House of York and the House of Lancaster, fought for the English throne. pitted two royal houses, York and Lancaster, against each other in 15th Century England. But the rivalry between the three York brothers, ambitious for power and wealth, tore the family apart.

Historians disagree about who killed who and how. But Shakespeare made the youngest into the story's villain. In his play³, Richard III drowns his brother George in a barrel of wine and murders Edward's sons in the Tower.

Culture clash. In 1532, 169 Spanish conquistadorsSpanish or Portuguese soldiers who travelled to the Americas looking for land and gold. stumbled into the largest empire in the Americas. The IncasA native South American people who lived in the Andes before the Spanish conquest. had powerful armies and experienced generals. But they were also divided by a bloody civil war between two brothers, Atahualpa and Huascar.

The Spanish used this to their advantage, taking Huascar's side in the conflict. After Atahualpa killed his brother, the Spanish swept through the empire as liberators.

The psychologist Alfred Adler argued siblings "strive for significance" and inevitably find themselves in competition. It is the fundamental rivalry that shapes our world.

But others have argued loving people like brothers is what holds society together. It is the principle behind many world religions and the motto of the French Revolution: liberty, equality, fraternity.

Yes: History is the story of men dividing their land, wealth and titles between their sons. No heir was a problem for the king. One son secured the future of the dynasty. Two or more meant trouble.

No: There is so much more to understanding history. For example, superior technology helped the Spanish defeat the Incas. And new diseases from Europe killed more people than the conquistadors.

Or... Sometimes. "The heir and the spare" survives in the British royal family, but the rest of society has changed. Power is distributed democratically and jobs are assigned according to merit.