Why are Rural People Vulnerable to Famine?

It is now generally accepted by researchers (although not by governments) that famines are caused as much by act of man as by act of god. But our understanding of famine is still quite rudimentary, and what little we know is rarely translated into policies to prevent or control famine. Few people would argue that we clearly understand what makes people vulnerable to famine, or that we can predict that one group will be vulnerable while another will not.