Choosing a running mate is one of the first major decisions that a presidential nominee makes; it’s an early window into his or her personality, priorities and campaign strategy. It’s … important, and the media will spend endless hours discussing the pros and cons of each potential pick. But the press typically overestimates the electoral impact of each running mate while missing his or her real influence. So, what do we really know about why vice-presidential candidates are picked and the effect they have on elections? Here is a user’s guide to the “veepstakes” process: the conventional wisdom, the research and what it means for 2016.

Home sweet home

Political geography plays a big role in presidential elections. With the Electoral College in mind, why wouldn’t presidential candidates use the second slot to attract votes from a crucial state or region?

Mitt Romney’s selection of Wisconsin Rep. Paul Ryan in 2012 looked like a geographically strategic pick. And, historically, balance between the North and the South, or the rural West and the urban East Coast, has been seen as desirable for a presidential ticket.

Despite the emphasis on geography in the media, however, the evidence for its influence on who gets picked as VP is more mixed. Jody Baumgartner, a political science professor at East Carolina University, finds that candidates tend to pick running mates from different regions than they themselves hail from. Baumgartner’s results also suggest that being from a competitive state increases the likelihood of being picked, at least a little, but there’s often not any obvious swing state-focused strategy to this: Think President Obama’s selection of Joe Biden (Delaware), or Al Gore’s selection of Joe Lieberman (Connecticut).

Size can matter: One classic study notes that, based on VP “short lists,” someone from a larger state — one with more Electoral College votes — is more likely to be chosen as a running mate.

Do vice presidential candidates help a ticket win their home state? Not really. A new book by two political scientists — “The VP Advantage: How running mates influence home state voting in presidential elections” — finds that these effects are “conditional and rare” and uses American National Election Studies data to demonstrate that even famously strategic picks, such as John F. Kennedy’s selection of Lyndon Johnson in 1960, did little to influence election performance. This is all consistent with previous research, which points to an unsurprising conclusion: Presidential elections are about presidents, not vice-presidents, and that goes even for residents of the running mate’s home state. Beware stories claiming a vice-presidential nominee will put his or her home state in play.

What it means for 2016: If the 2016 nominees take to heart the idea that there isn’t much of a VP home-state effect, it frees them up to think about other factors (which we discuss later).

This might mean that Hillary Clinton can forgo fretting about whether there are any good Florida Democrats to bring on the ticket, or vetting New Hampshire Sen. Jeanne Shaheen (a frequent flyer on veepstakes lists) or Ohio Sen. Sherrod Brown — unless those are really the people she wants to consider. And it’s hard to imagine a candidate as unconventional as Donald Trump being strategic in this regard anyway, but he might be even more disinclined than usual to make nice with John Kasich or Marco Rubio.

Then again, the structure of the Electoral College makes political geography important, and the research is not conclusive. If the election looks like it will be close, the candidates could still try to use their VP picks to gain a tiny advantage in a crucial state.

Left/right

Here’s another conventional wisdom favorite: Presidential candidates should use the VP pick to move to the center! Or shore up the party base! Then there’s the trope that candidates could truly unify the nation by crossing party lines in their running mate choice. (That’s worked really well in the past!)

Ideology is notoriously tough to measure. Many of the most influential studies rely on nonquantitative assessments, and, among modern presidential candidates, ideological balancing doesn’t seem to count for much when picking someone for the second slot on the ticket.

This isn’t too surprising, especially recently, as parties have become more ideologically consistent within their own ranks. Republicans tend to be some shade of conservative, Democrats some shade of liberal. Palin and Ryan were probably chosen with some idea of energizing the base (and there’s some evidence it succeeded with Ryan, at least). But Democratic running mates such as Biden, John Edwards (2004) and Al Gore (1992 and 1996) weren’t generally perceived as wildly different, ideologically speaking, from the presidential candidates who chose them.

We can use DIME scores, which are derived from campaign donations, to compare ideology across candidates who have served as legislators and governors. It’s not enough observations to really call it a pattern, but there’s enough to hypothesize that the two parties have different strategies.

All but two Democratic candidates picked running mates who moved the ticket to the center. In 1988, for example, Michael Dukakis picked a running mate who was well to the right of his own positions, Lloyd Bentsen. These scores also suggest that Biden was much closer to the center than Obama. For Republicans, the opposite is true: Most vice-presidential candidates have been more conservative than the presidential candidate.

What it means for 2016: Ideological questions are likely to be more pressing for Clinton than for recent Democratic nominees. With Bernie Sanders winning 21 contests and nearly 43 percent of the popular vote so far, appealing to his supporters may be important for Clinton in a way that defies previous research. Someone like Sen. Elizabeth Warren or Brown could act as a bridge between Clinton and Bernie’s supporters (or, although it’s very unlikely, Clinton could tap Sanders himself).

Trump’s situation is unique, too, given the stated opposition to his candidacy by other Republicans dubious of his conservative credentials. A well-known conservative figure could assist, but it’s not clear who that would be (Ted Cruz, for instance, might have some difficulty playing second banana). Newt Gingrich has been mentioned, and his conservative credentials are pretty clear.

Although the published research doesn’t turn up much about the importance of ideological balancing, the ideological scores above show that Republicans have pretty consistently been attentive to the party “base” when choosing a running mate. Democratic nominees do not mirror this tendency. If this changes in 2016, it might just be a reflection of the weird year we’re having. Or it might signal a larger shift in party strategies and priorities if Democrats have become more ideological.

A lot of white guys

The first thing to note is that the conventional wisdom on vice-presidential demographics has shifted. No longer seen as a major risk, picking someone from an underrepresented group is instead seen as a way to make history while broadening the ticket’s appeal. Hence we see calls for Clinton to choose an African-American running mate; or another woman to double down on Trump’s lack of appeal to female voters; or the first Latino running mate (though there is some disagreement about which one might be ideal). Or, perhaps Trump can overcome his weakness with Latino voters by a smart VP choice.

Gender: There has been a lot of talk about balancing the ticket demographically, but very little of it in practice. Only two women have ever been selected, and the overwhelming majority of running mates have been white male Protestants. The two women who were selected as major-party running mates — Sarah Palin by John McCain in 2008 and Geraldine Ferraro by Walter Mondale in 1984 — drew a great deal of attention, but their overall impact remains unclear. There’s not a lot of evidence that putting a woman on the ticket does much to pull in female voters who, like their male counterparts, are largely driven by their party identification.

There is ample evidence, however, that women candidates receive less-than-stellar treatment in the media. A study published in 2015 found a great deal of sexism in the coverage of Palin’s candidacy, including lots of references to her looks and Barbie-related nicknames. And a 2013 book by Stacy G. Ulbig found that Palin’s coverage was a big improvement over the way the media treated Ferraro. Ferraro’s treatment is a very low bar: Questions were raised, for example, about whether foreign leaders would try to take advantage of her.

Religion: Again, there hasn’t historically been a ton of diversity in the religious affiliation of vice-presidential nominees. Joe Biden was the first Catholic to be elected vice president, though four others were nominated (William Miller in 1964, Ed Muskie in 1968, Sargent Shriver in 1972 and Ferraro in 1984). Joe Lieberman, the former senator from Connecticut and Al Gore’s running mate in 2000, is the only non-Christian to appear on a modern major party ticket (he is Jewish).

Although the media widely discussed Lieberman’s Jewish identity at the time, there is not a lot of research on what effect his religion had on the vote. Two studies suggested that negative reactions were limited and that “the American public appeared warmly disposed toward both Jews and Lieberman” based on evidence from a 2000 survey. Neither of these studies directly addresses the question of whether Lieberman’s presence on the Democratic ticket increased the Democrats’ share of the Jewish vote.)

Race: There has never been a black or Hispanic major-party vice-presidential nominee, and there has been just one non-white VP candidate: Charles Curtis, who was selected as Herbert Hoover’s running mate in 1928 and had significant Native American heritage, including a tribal enrollment.

It’s just hard to pinpoint the effect of having a VP nominee from an underrepresented group — most clearly with race, but also with religion and gender. It’s also hard to separate how running mates’ demographic characteristics influenced voters from everything else.

Keep that lack of precedent in mind: Any story making bold predictions about the electoral effect of picking a woman or Hispanic running mate is working with scant evidence.

What it means for 2016: One of the big questions of the election is one that the existing research can’t really help us with: How would it play out if Clinton were to choose another woman as her running mate? Say, Warren. We’ve had about 100 major party tickets featuring two men, but the female half of the population has never been represented that way. The best we can do is speculate. There is no shortage of evidence of sexist media coverage of female candidates, and we could certainly expect to see that continue. But Ulbig’s research suggests that the historic nature of such a ticket might generate media attention, and it might bring something fresh to an otherwise conventional, “establishment” candidacy.

Or, should Clinton increase the demographic quotient by picking someone like HUD Secretary Julian Castro? A strategy that relies on reaching out to moderate Republicans, on the other hand, may see her go for a more conventional white male such as Tim Kaine of Virginia or, again, Sherrod Brown.

For Trump, like everything else about him, the strategy is hard to imagine. He has some major demographics-related problems — women and nonwhite people don’t like him — and you could imagine a normal candidate trying to ameliorate those issues through their running mate (the lack of empirical evidence suggesting such a strategy would work notwithstanding). But diversity has not seemed important to Trump in other areas, such as potential Supreme Court nominees. Maybe Rubio could help him reach out to Hispanic voters (at least more than a taco bowl would). Picking former VP nominee Palin, as some have suggested, seems likely to backfire, based on the available evidence from her 2008 run.

The resume

In each election cycle, we hear a lot about political experience in terms of ticket balancing. Legislators pick executives and vice versa. Relatively inexperienced candidates (as Obama was in 2008) will choose someone who’s been around for a while (like Biden). When we think about balancing the other way — grizzled Washington veterans gravitating toward fresher faces on their tickets — the line between age and experience starts to blur.

Research suggests that governing experience matters — the more, the better. In a 2008 article, Jody Baumgartner found that while many commonly cited factors — region, ideological balance — make little difference in the selection of a running mate, years of experience, at the national level and below, improve the chances of being selected for the ticket. Other research finds a similar pattern: Political scientists Mark Hiller and Douglas Kriner find that governing experience has been an especially important selection criterion since the 1970s, as competence to succeed the president has eclipsed salving party divisions as a main purpose of VP selection.

What it means for 2016: For Clinton, the question of governing experience has already come up once. HUD Secretary and former San Antonio Mayor Julian Castro has been suggested for her VP, and questions have been raised about his experience. Some other possibilities on her list include Cory Booker, a first-term U.S. senator from New Jersey. Given her vast experience in government, however, this factor may be less relevant.

For Trump, of course, the calculus is quite different. Normally, we might expect that the insurgent Republican nominee would select a running mate with enough experience to balance out his own lack of it. But to his supporters, part of Trump’s appeal seems to be defiance of party leadership and political expectations. Choosing a conventional politician with years of experience in Congress (for example) would undercut that message.

There’s often a big gap between what we think matters and what factors seem to influence candidates’ choices, and how voters react to them. The research isn’t always consistent, but a few patterns are clear: Experience matters. Female running mates garner media attention, but not all of it is positive. Home-state advantage may exist, but only in certain cases and even then at the margins. And ideological balancing, along with other forms of ticket balancing, is a much smaller part of the modern VP selection process than many media accounts would suggest.

Nevertheless, the 2016 caveat remains. With the first woman likely to head the Democratic ticket, and … whatever Donald Trump’s candidacy is, it’s hard to know how much the old rules apply. That, of course, won’t stop the media.

CORRECTION (May 31, 11:52 a.m.): A previous version of this article incorrectly stated that there has never been a non-white vice-presidential nominee of a major political party. There was one in 1928: Sen. Charles Curtis of Kansas, who was Herbert Hoover’s running mate, had significant Native American heritage, including enrollment in the Kaw nation. He served as vice president from 1929 to 1933.