Over the last 50 years, there have been efforts on behalf of the US government to simplify public legal documents for the benefit of society at large. However, there has been no systematic evaluation of how effective these efforts–collectively referred to as the “plain language movement”–have been. Here we report the results of a large-scale longitudinal corpus analysis (n $\approx$ 225 million words), in which we compare every law passed by congress between 1951 to 2009 (as well as concurrent resolutions and proclamations), with a comparably sized sample of English texts from four different genres published during the same time period. We find that laws remain laden with features associated with processing difficulty–including center-embedding, passive voice, low-frequency jargon and capitalization–relative to each of the four baseline genres of English, and that the prevalence of these features has not meaningfully declined since the onset of the plain language movement (in some cases, their prevalence has increased). These findings suggest that top-down efforts to simplify legal language have thus far remained largely ineffectual, despite the apparent tractability of these changes, raising and informing difficult questions of law and public policy.