It should be a no-brainer that happy employees are more productive, but in case you need proof, a new study proves it.

New research from Oxford University’s Saïd Business School, in collaboration with British multinational telecoms firm BT, has found that happy employees are 13% more productive than unhappy ones.

According to a media release, the link between happiness and productivity has often been discussed but this is the first study showing causal field evidence for the relationship (which is kind of surprising since this seems like the sort of thing people to studies on). See also: 8 tips for getting to work on time every day

“There has never been such strong evidence,” said Jan-Emmanuel De Neve Of Saïd Business School at Oxford.

The research was conducted in British telecoms firm BT’s contact centres. Workers were asked to rate their happiness on a weekly basis for six months. They used one of those surveys containing five emoji buttons representing states of happiness, from very sad to very happy (I didn’t see it but I’m guessing frowny face, less frowny face, “meh” face, slightly smiley face, very smiley face like you see on your way out of stores). The survey was sent by email.

The researchers tracked data on attendance, call-to-sale conversion, customer satisfaction, and the worker’s scheduled hours and breaks. They then collated this information with administrative data obtained from the firm on worker characteristics, work schedules, and productivity.

They found that happy workers are more productive than their less happy counterparts. It wasn’t that they worked more hours than the others, they were simply more productive within their allotted work time.

“We found that when workers are happier, they work faster by making more calls per hour worked and, importantly, convert more calls to sales,” said De Neve.

Recent research on happiness has found that doing paid work ranks pretty low on the list of activities that make people happy.

“There seems to be considerable room for improvement in the happiness of employees while they are at work,” said De Neve. “While this clearly in the interest of workers themselves, our analysis suggests it is also in the interests of their employers.”

The study also factored in local weather conditions and uncovered a clear negative relationship between adverse weather conditions and the happiness of the workers.

You can’t control the weather but you can put a lot of effort into keeping your people happy. Not only will this make them more productive, which, again, frankly should be obvious, it will also go a long way towards attracting top talent. Happy workers tell their friends about their amazing workplace. So do unhappy ones.

Now, how can you make your employees happy?