Walker does not cite any studies that support this assertion anywhere in the book. I actually literally had to go to Google and try to find a study that Walker mentioned simply based on like the keywords and sample sizes, but without them being mentioned anywhere in thebook. And I think that what happened was just that Walker was trying to perhaps remember whatever impressions he had from his professional career and just throw them down. He essentially just never got back to figuring out whether they're supported by the literature or not.