Twenty Democratic presidential candidates will participate in the party's first debates in Miami on Wednesday and Thursday. (Reuters File Photo/Reuters)

Candidates for the Democratic Party nomination will square off Wednesday night in the first of two televised debates. Pundits will focus on what the candidates say. But how politicians speak can be just as important. If you watch, keep your eyes on the candidates’ mannerisms — from hand gestures to vocal inflections. Why? Because politicians’ nonverbal behaviors give us clues about which issues are especially important to them.

This applies far beyond campaign debates. In recent work, we examine the emotional intensity of lawmakers’ floor speeches in the House of Representatives. Female politicians are more likely than men to talk about women, and they do so with greater emotional intensity. And that emotionally intense speech has consequences for male politicians’ behavior on women’s issues.

Here’s what you need to know about how politicians speak.

1. Here’s how we did our research

Over decades, political scientists have developed measures to capture politicians’ ideological positions, checking how behaviors like voting, campaign donations and speech content enable us to position lawmakers along the liberal to conservative (from left to right) scale. But knowing where members of Congress stand on particular issues doesn’t tell us how committed they are to those policy positions.

Drawing on psychology research on emotions and vocal communication, we argue that speakers' emotional intensity is reflected in minor changes in vocal pitch. When a speaker is emotionally activated, his or her muscles — including vocal cords — tense, which leads to slightly higher-than-average vocal pitch when speaking. Because changes in vocal pitch are hard to control, they provide a good measure of speakers’ emotional investment in issues under debate.

To detect lawmakers’ vocal pitch, we collected audio and text data from all floor speeches — nearly 75,000 — delivered in the House of Representatives between 2009 and 2014. Using these, we estimate the average vocal pitch across each lawmaker’s speeches to form a unique baseline for each member of Congress. We can then determine when lawmakers speak at a vocal pitch higher than their baseline. We use those deviations to identify issues on which legislators are especially emotionally invested.

2. Women talk more about women’s issues and with greater intensity

Using our measure of emotional intensity, we found differences between male and female lawmakers when they delivered floor speeches about women’s issues. Not only do congresswomen speak more often about women, but they do so with greater intensity.

For example, two Democratic representatives — Danny K. Davis of Illinois and Wisconsin’s Gwen Moore — speak about the importance of education to women. To watch the clips, just click on the speaker’s name. Moore’s delivery and heightened vocal pitch show more intensity than does Davis’s comparatively monotone delivery. We find similar patterns across our data set.

3. More emotional speech reflects greater political commitment

This intensity accurately reflects how members vote.

Using legislative scorecards from 24 prominent women’s interest groups as reported by Project Vote Smart, we discovered that female lawmakers’ more emotionally intense speech about women was consistent with their votes on issues related to women.

Though on average congresswomen speak with higher-than-baseline pitch when referencing women, female members of Congress differ among themselves. The female representatives who speak with the greatest intensity about women receive significantly higher scores from women’s interest groups. Congresswomen who spoke with the most intensity about women tended to vote 21.17 percent more in line with women’s interest groups than did congresswomen who were the least intense when speaking about women.

4. Men respond to women’s emotionally intense speech

As the amount and intensity of women's speech increases, male members of Congress seem to react by talking more (and more intensely) about women as well.

Consider, for example, the nine speeches delivered by congresswomen on Oct. 7, 2009, about breast cancer awareness. Some of these speeches, like those delivered by Reps. Sue Myrick (R-N.C.) and Lynn Woolsey (D-Calif.), highlighted important facts. Others shared personal narratives. Rep. Shelley Berkley (D-Nev.), for example in the clip shown below, talked about the first time she learned her sister was diagnosed with breast cancer. In each speech, the congresswomen are clearly emotionally invested in the issue they are discussing, as you can hear in their heightened vocal pitch.

It’s hard to know for sure that these congresswomen influenced their male colleagues. But Reps. Bill Foster (D-Ill.), Steve Driehaus (D-Ohio) and Joseph Cao (R-La.) all gave speeches that were unusual, compared to their previous floor speeches. Driehaus in the above clip talked about his grandmother for the first time. Foster first mentions his sister. Cao spoke about a woman from his district who was diagnosed with breast cancer, something he had not done in any previous floor speech. Each of these male members of Congress voted for the legislation advanced by the above congresswomen.

We find a similar pattern in our data. When many female representatives take to the floor to speak with intensity about women, male speakers are more likely to vote in line with these female speakers than their past voting history would predict. In other words, electing female representatives who vocally champion women can advance women’s interests in Congress.

If you watch the Democratic debates, pay attention to how candidates speak about the issues. Candidates’ nonverbal behavior can inform us about their emotional commitment to policy issues. All voters have to do is listen.

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Bryce J. Dietrich is a research fellow at Harvard University’s John F. Kennedy School of Government and an assistant professor of social science informatics at the University of Iowa.

Matthew Hayes is an assistant professor in the department of political science at Rice University.

Diana Z. O’Brien is an associate professor in the department of political science at Rice University.