We Have a Social Duty to be Reasonably Altruistic

This is an awful story: A man needs a bone marrow transplant to save his life, and the only suitable donor is his sister. But she won’t do it. Now, he will almost surely die.

The law can’t force anyone to be altruistic, of course. It would be wrong to take her bone marrow by force. But doesn’t she have a social duty? This isn’t like asking for a kidney, which anybody would be justified to refuse. It is bone marrow. Extraction isn’t fun, of course, but it is an extremely low risk procedure and a life is literally at stake. A stranger would have a moral obligation to help, it seems to me. We owe each other that much. So much more, a sister.

It strikes me that unless there was an overriding health issue that compelled the sister to refuse to save her brother’s life, she should be shunned socially. What say y’all?