No wide spread studies have been done as far as I know, but it does happen. The fact that the very large numbers means a high probability of this happening and your prove me wrong statement does show bias on your part. https://www.sciencedirect.com/science/article/pii/S0261379414000973 https://www.washingtontimes.com/news/2017/jun/19/noncitizen-illegal-vote-number-higher-than-estimat/