Teacher strikes swept the United States in 2018, from West Virginia to Oklahoma, Colorado, Arizona, North Carolina and beyond.

The demands varied across states, but a raise in teacher pay was central to each. Now politicians are proposing large increases in teacher salaries at the state and Sen. Kamala Harris’ has called for an average raise in teacher pay of US$13,500 – or more than 20% – in her first term, were she to be elected president.

What would a raise in teacher salaries accomplish? As one who has studied the economics of education around the world, here’s what I know about what recent research on different countries that have boosted teacher pay.

What do teacher salaries do?

People might think that raising teachers’ salaries will result in better learning for children. One reason is that higher teacher salaries might increase teachers’ effort. When teacher salaries are low, teachers may get a second job, reducing their energy and effort at teaching. Indeed, there are reports of teachers taking second jobs in Idaho, Pennsylvania, Texas and elsewhere. Higher teacher salaries might also increase learning if they draw more of the best and brightest college graduates into teaching.

Recent evidence from more than 30 countries shows a clear link between teachers with higher cognitive skills and subsequent student performance. And which countries have teachers with higher cognitive skills? Countries with higher teacher salaries. But the story is not that simple.

Higher teacher salaries don’t boost effort

Fifteen years ago, Indonesia embarked on a policy experiment that shed new light on how salaries affect teacher effort. Over the course of 10 years, Indonesia raised salaries by more than a quarter for a subset of teachers. They randomized the roll-out across schools, which allowed researchers to compare schools that got the raises early on to schools that wouldn’t get the raises until much later. The result? Teachers were happier, and they were less likely to hold a second job. The reform initially decreased teacher absenteeism, but that effect disappeared by the second year. Student learning remained unchanged.

In Uruguay, increasing teacher salaries by about 25% for teachers working in poor neighborhoods had little to no impact on student learning. Similar studies show the same for programs in African countries, like the Gambia and Zambia.

Raising salaries attracts and keeps good teachers

In Texas, increasing teacher pay reduced turnover, which in turn increased student performance. Likewise, national studies from the U.S. and the U.K. also find that students do better when teachers have relatively better wages.

Studies from Latin America have looked specifically at the pull factor of higher wages for civil servants – of which teachers are a subset. In Brazil, higher wages for civil servants drew more educated candidates into the service. In Mexico, higher salaries for civil servants attracted more candidates who were more conscientious and who had higher IQs. But higher salaries also attract less qualified candidates. In education, one challenge is selecting those candidates who will go on to be great teachers, which brings us to the topic of higher standards for teachers.

Reforms beyond just salary increases are needed

What countries that have made large gains in learning have shown is that combining salary increases with other critical reforms is the way to success.

Setting higher standards to enter the teaching profession is a way to both pay teachers what they’re worth while making sure the very best candidates are teaching. Finland and Singapore, two countries known for high performance on international tests, have highly competitive entry into the teaching profession. In both countries, a small fraction of applicants to teacher training schools are accepted, allowing teacher training schools to only accept those applicants with excellent academic credentials. By contrast, a recent study of teacher preparation graduate programs in the U.S. found that fewer than half required a 3.0 GPA.

Ecuador provides a clear example of how increasing teacher selectivity can lead to gains. Ecuador doubled teachers’ starting salaries in 2009. At around the same time, it introduced a national hiring exam and teacher evaluation systems, and it made getting into teacher training colleges and subsequently getting a job as a teacher more selective. The country also instituted incentives for high performing teachers. Ecuador went on to register the highest student literacy gains of any country in Latin America on regional tests conducted between 2006 and 2013.

In other countries, the key reforms may be different.

Brazil registered large learning gains in the first decade of this century after a series of reforms in the 1990s. These reforms increased teacher salaries while also increasing the educational requirements to become a teacher, expanding in-service support for teachers, ensuring more financing for rural schools, and later, introducing better measurement and publicity around student learning results.

Kenya recently saw student learning rise with a nationwide program that included detailed teachers’ guides, professional development and coaching for teachers.

The optimal education system

In a recent study, the World Bank highlighted how many education systems seem to be stuck in a low-learning trap, where teachers and schools lack both the support and the motivation to give students what they need. Low teacher salaries, together with inadequate support for teachers and little selectivity in teacher preparation, can keep U.S. schools far below their potential. But increased pay is not enough. As experiences from around the world show, higher pay must be accompanied by an array of other reforms – ranging from increased selectivity into the field to more mentoring and coaching to help teachers already in the field give their best to our students.

In addition to his work at the Pardee RAND Graduate School of Public Policy, David Evans is also a Senior Fellow at the Center for Global Development.