As the year comes to an end, you might be in search of ways to celebrate your employees' hard work and get them excited about the year ahead. Maybe you're thinking about a company party, a gift, or even a year-end bonus. But according to behavioral economics research, money and gifts are not necessarily the most effective ways to motivate your employees. In fact, employees are more motivated when they can do meaningful work that is acknowledged. Here are four things that will motivate your employees more than a pay raise.

1. Challenge your employees

In a Duke University experiment, researchers gave half of their subjects instructions for building a difficult (and ugly) origami object. The other half didn't get any instructions. Both groups had an audience watching them build. When the objects were complete, the group who had instructions valued their objects five times more highly than their audience did. The group who didn't have instructions valued their objects even more highly than their audience did, who valued their objects even less than the other audience had valued the other group's.

The more effort people put into their work, the more invested they are and the more they value the outcome. Notably, the value we place on our work isn't affected by the value placed on it by others (in this case, the audience).

When delegating a project to employees, ensure that they find it to be a healthy challenge, and then empower them to lead the effort.

2. Create projects with impact

In an experiment at the University of Michigan's fundraising scholarship call center, researchers connected funded students with callers to have a 10-minute conversation. A month later, callers were spending 142 percent more time on the phones than before and revenue had increased by 171 percent.

Though callers denied that they had been impacted by the students' visit, there was clearly some subconscious force that was driving them to be more involved than before.

For any task an employee might tackle, help him or her understand why it is important and the impact it will have on the organization. This will set context for the way in which his or her work will inform and support that of others.

3. Share the final product

In one study, a Duke behavioral economist asked two groups of participants to build Lego Bionicle characters. Both groups were paid for each Bionicle they created, but while the first group got to see their creations stored at the front of the classroom, the second saw their creations destroyed as soon as they turned them in.

The first group made 11 Bionicles before they quit while the second made only seven.

Especially on large or long-term projects with many work streams or moving parts, it's important that people can see the meaning of their contribution to the overall vision. Find frequent ways (as frequent as twice a month) for employees to see how a project is progressing and to demo their work.

4. Acknowledge your employees' work

In an MIT study, researchers asked students to complete several rounds of an exercise in which they were to identify letter-pairs on a page with random letters. Students were paid to complete this task, but they were paid less with each new round. Students were split into three groups. The first group wrote their names on their papers, and the experimenter looked them over and said "Uh huh" before putting them down. The experimenter put the second group's papers face-down in a pile without looking at them, and shredded the third group's papers before their eyes.

People whose papers were shredded needed twice as much money as the group whose work was even mildly acknowledged. The second group needed almost as much money as the shredded group. Failing to acknowledge someone's work is almost as good as ripping it to shreds in front of them.

Set up milestones and check-ins with your employees to explicitly acknowledge and discuss their progress. Whether you're providing feedback or just listening, your interest will propel your employees forward.

While perks and financial compensation may make employees feel valued, that value does not necessarily translate to their feelings about their work. Instead, enable employees to take ownership of projects that they find meaningful, and then acknowledge their contribution. And they will thank you back.