The more than 100 social impact bonds launched in the past eight years have mobilized more than $400 million in total and touched more than 700,000 lives in two dozen countries. Is that a lot, or a little?

In roughly the same time period, "green bonds" issues have reached more than $120 billion per year. Social impact bonds have been bedeviled by complex structures and measurement difficulties, not to mention the thorny social problems themselves.

Read the full article about the true value of social impact bonds by David Bank at ImpactAlpha.