If you want a child to miss fewer days of school, it's not enough to tell their parents how many days they've been absent.

According to behavioral science research, you also need to compare their attendance record to that of a typical child.

Across the United States, 17 cities have started embracing that wisdom by sending "nudge letters" to families of kids who are chronically absent (those who have missed an average of two days a month).

Nudge letters get their name because they utilize "nudge theory," which states that human behavior is driven in large part by subtle cues rather than the exercise of willpower.

A lot of nudges involve comparing someone's poor or unfavorable behavior to someone else's. Since under-performers sometimes think they're close to the average, making the gap explicit can motivate them to do better.

For instance, people vote more often when they learn how their record compares to typical voters, and they use less energy at home when they see what their neighbors pay each month.

Within the last several years, cities such as Chicago, Philadelphia, San Mateo, and Tacoma have all started sending home nudge letters that compare a student's absence tally to the average — most to positive effects. In one Tacoma district, for example, 62% of students had improved attendance records, The Seattle Times reports.

Broader research also supports the effectiveness of nudge letters in schools. Data from In Class Today, a company that offers services to reduce absenteeism rates, has found kids attend school 11-15% more often when parents learn how their kids stack up to the average.

A typical nudge letter includes a notice alerting parents to the child's chronic absenteeism. For many parents, this comes as a surprise, behavioral scientist Todd Rogers, who also does work with In Class Today, told the Seattle Times. Often, parents aren't keeping track of how many times a kid has been absent. And even if they are, they probably don't have a clear sense what rate is considered normal.

"Getting this discrete, actionable information to parents increases student achievement," Rogers said. "And parents want more of it once they get it."

Nudge letters aren't the only effective approach to curbing absenteeism. In some communities, particularly those with a high percentage of low-income students, installing a washer and dryer in a school has been shown to encourage kids to show up more often. In August 2016, Whirlpool donated a washer and dryer set to 17 schools in St. Louis, Missouri and Fairfield, California. By the end of the 2015-2016 school year, over 90% of kids came to school more often than they did before the program began, with an average increase of 6.1 days per year. Kids also participated in class more.

Rogers estimates the cost of nudge letters is minimal — about $5.50 per household accounting for production and labor, according to a study he conducted in Philadelphia.

Nudges have their limits, of course, since letters and washing machines can't completely fix a systemic problem. But if a family is stuck in a destructive pattern, a nudge could provide a cheap and gentle push them toward a more productive set of behaviors.