What a bleak day on Wall Street. Although the financial contagion long ago spread beyond subprime mortgages, it’s worth remembering that this all began when lenders decided that millions of people could afford loans that were actually unaffordable, at least over the long-term. Why did people accept these onerous loans? Why did they borrow more than they could possibly repay? The answer, I think, returns us to a basic cognitive flaw, built into the brain.