All too often, an ex-inmate walks out of prison with the exact same problems he or she walked in with: lack of skills, lack of support, no job. And they end up re-offending and back in jail. It's an expensive problem to fix, but it's a much more expensive one to ignore. A director at Social Finance in London, Toby Eccles explores the arbitrage between those two options.

In 2010, his pioneering Social Impact Bond allowed private investors to support a UK program targeting ex-prisoners who served short sentences (the limited government funding only goes to ex-inmates who served long terms). The £5m scheme, funded by 17 investors, supports training and support for 1,000 ex-inmates; if they re-offend less than a control group, the government will pay investors back, plus interest, through the savings accrued by achieving the program's targets.

More such bonds are now being tried across the world, including in New York City and Massachusetts (both addressing recidivism), and extended to new fields such as development. Eccles founded Social Finance in 2007, and he oversees all of the firm's social impact bond work, where, he says: "We are incentivised to work with the complicated and with those willing to change." "We are incentivised to work with the complicated and with those willing to change."