Helping hand
According to the theory, people or animals can pass on their genes by helping their relatives. That means they should be more willing to help brothers or sisters, who share half their genes on average, than more distant relatives who share less.

But relatives also have a nasty habit of competing with you for resources, be they chicks squabbling over food, or heirs fighting for the throne. And this is where the flip side of Hamiltons rule comes in.

If you kill two brothers, say, youve essentially wiped out your genetic identity rather than preserving it for future generations, says McCullough. "Then you have really damaged yourself in an evolutionary sense. So you can eliminate relatives but only up to a certain level."

But it turns out no king or queen, from Edward III who succeeded to the throne in 1327 to Elizabeth I who died in 1603, killed enough relatives to wipe out the equivalent of their own genetic inheritance (see graphic).

Edward IV, who reigned in the late 15th century, was the worst offender, executing his brother, George, Duke of Clarence, and five cousins, including Henry VI and Edward, Prince of Wales. Yet even Edward IVs victims shared only two-thirds of his genes in total.

"I was astounded. We thought that at least two or three would violate [Hamiltons rule] because some very close relatives were killed," says McCullough. "They had no theory of genetics at the time so they were simply operating under their own set of rules," he says. "As it turns out, it is in accord with scientific expectations."

Yet the art and science of medicine has limits. A few times in the operating room I peered through the window of those limits at deaths inevitability, as powerless to intervene as a 16-year-old confined to the backseat of a car in the middle of the desert. But in 1970 my limitations were relative to what I had yet to learn.
After my medical training they were for the most part absolute. The decision trees I adopted branched with lifesaving options, including one final, seldom-rehearsed limb available to all physicians: let go, relinquish hope, accept deaths tenacious hold. Coupled to a physicians obligation to provide medical care to the best of his or her ability is the burden to decide when nothing more can be done. I was fortunate. The few times I had to make that decision were within the security of a well-equipped and well-staffed hospital. There I found solace in technology and a consensus of educated opinions. Now I see a more complete picture of my fathers grief that August day.