America's prison problem;

The US locks up more people than any other country in the world, spending over $80 billion each year to keep some two million prisoners behind bars.

Over the past three decades, tough sentencing laws have contributed to a doubling of the country's prison population, with laws like the 'Three Strikes and You're Out' mandating life sentences for a wide range of crimes.

Why does the US put so many people behind bars and what lies behind California's new push for leniency? But a clear sign that Americans are rethinking crime and punishment is a voter's initiative on California's November ballot called Proposition 36 that seeks to reform the state's three-strikes law.

Some 27 states have three-strikes laws patterned after California's version, which was one of the first to be enacted in the country.

Activists who are campaigning to change the three-strikes law in California are also trying to raise awareness about conditions inside prisons. They are targeting the use of special security units at maximum security prisons