I'm not saying this isn't true (there's a lot of evidence to support this claim), however, if you read the actual report the Guardian is reporting on, all but one of their sources are Vox, NYT, WaPo. It wouldn't be controversial to say that these outlets lean left, and therefore would draw conclusions along those lines. Why didn't the Brennan Center report look at the studies directly? I think that taints the sources a bit.