It's a new research finding likely to be protested by 5-year-olds everywhere: People who eat more vegetables and fruits are significantly happier than those who eschew such foods. 

Dartmouth University's Daniel Blanchflower looked at the eating habits of 80,000 British citizens. He, alongside two British researchers, saw that mental well being -- satisfaction with one's life on a scale of one to 10 -- rose alongside each serving of produce consumed daily.

Those who consumed eight or more servings of vegetables daily rated themselves 0.27 points happier, on average, than those who had no servings to speak of. The effect was strongest for those who ate seven servings of produce daily. After that, grabbing a handful of carrots doesn't seem to have much of an effect.

This relationship held true when the researchers controlled for education and income. Blanchflower told me, in an interview, that his students have found similar results here in the United States.

"Our response was 'Holy moly, it's there in the data,'" Blanchflower says of the paper.

There is one big, outstanding question that he acknowledges: What's the causal relationship? It could be that eating vegetables make people happy -- or that happier people tend to chow down on more salads.

"It might be that we just have all these vegetarians that are richer or happier," Blanchflower says. "There are definitely issues of causality. At the same time, I think what we've done here is establish correlation. I don't think we expected to see the relationship we did. The reason you're calling me is because this was unexpected."

As Blanchflower notes, it's a bit difficult to design an experiment that would tease out these different influences. You would need, as he puts it, "a controlled, random experiment where you randomly make people eat carrots."

The follow-up research he's most interested in is understanding how vegetables -- and how happiness -- effect people physically. "Happiness does seem to correlate with certain physical things and biomarkers," Blanchflower notes. "We know that happy people have lower pulses, for example. So maybe that's a crucial path. It wouldn't surprise me."