请把下面的文档总结为3句话。 prisoners in the US rose from 200,000 at the end of the 1970s to about 2.2 million in 2008. Around 2010, various states began passing criminal reform legislation to curb the growth of the prison population. As of 2020, the US prison population was around 2.1 million, a slight decrease from the peak but still drastically higher than the 1970s level.
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美国的监禁人数从1970年代末的20万增加到2008年的约220万。2010年左右，各州开始通过刑事司法改革立法以遏制监狱人口增长。截至2020年，美国监狱人口约为210万，虽然较峰值有所下降，但仍远高于1970年代的水平。
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