What do Democrats say in poll after poll that they want in a 2020 presidential candidate? Someone who can beat Donald Trump. In the abstract, that makes plenty of sense. The only problems? No one really knows what voters will do in the primaries.

And there’s simply no good way to know which candidate will do best in the general election.

At this point, don’t put much faith in head-to-head trial heats against Trump. Those really just reflect name recognition. That’s why Joe Biden consistently does well, and candidates like Kamala Harris or Elizabeth Warren typically don’t appear as strong. People don’t like to vote for politicians they haven’t heard of, and a lot of voters either have never heard of, or know little about, most of the roughly two dozen Democrats running for president. Granted, a very unpopular but well-known candidate wouldn’t poll well, so the polls aren’t entirely meaningless. But there’s just no way to project them forward.

The findings from political science on electability are sparse. Most of the research is about ideology, and generally most political scientists would say that a candidate perceived as a moderate would, all things equal, do better than one perceived as an extremist. And while it’s safe to guess that Trump is going to call any Democratic opponent a socialist, it’s reasonable to assume that the one candidate who actually calls himself a democratic socialist, Bernie Sanders, is more likely to be thought of as one. To be fair, there’s at least some evidence that extremism may not hurt general election candidates.

There’s also some research on demographic characteristics of the candidates. For example, there’s little evidence that women candidates face a penalty in most United States elections — but there’s been only one election with a woman as a major-party presidential candidate, so it’s impossible to know whether there’s a penalty at that level. Hillary Clinton did receive more votes than Trump, and generally hit or exceeded “fundamentals” predictions based on the economy and other noncandidate factors; then again, you could argue that Trump should have severely underperformed such predictions. Political scientists have also estimated that Barack Obama paid an electoral penalty for being black in 2008. Again, there’s no real way to know if that would continue or dissipate for a potential second black president or a first Latino one, for example.