It looks like it was from The Economist and YouGov - I falsely remembered it being Pew. Here's a WaPo article with a summary and a link to the raw data:
https://www.washingtonpost.com/news/rampage/wp/2016/12/28/am...
https://www.washingtonpost.com/news/rampage/wp/2016/12/28/am...
Do you think it's mostly from misleading mainstream news stories? Or from smaller outlets (social media, celebrities, memes, etc)?