A handful of the nuts, even as an ingredient in cakes, can transform sullen adolescents into happier bengs, researchers have found.
It improves mood by up to 28 per cent but only in men.
And that has left boffins baffled as they have no idea why it does not work on young women too.
Walnuts have previously been hailed as a superfood with Omega Three to help ward off cancer and heart disease, prevent blood clots, battle diabetes and reduce nausea in pregnancy.
Now they have been found to lift moods, thanks to feelgood vitamins and acids including vitamin e, which protects ageing cells and melatonin, which controls the body clock.
However, in an experiment on students by nutrionists from the University of New Mexico in the journal Nutrients, it only seemed to work on men.
Research leader Professor Peter Pribis divided 64 students aged 18-25 into two groups and gave them three slices of banana bread every day for 16 weeks.
Half had banana bread containing a handful of walnuts and half had the treat without the nuts.
Specialist questionnaires were used before, during and after to measure moods including tension, depression, anger, tiredness, energy and other states of mind.
The male students who ate the walnut-based banana bread showed a 28 per cent average improvement on their mood scores at the end of the trial.
Those on the walnut-free bread and all the female students in the trial showed no change.
Professor Pribis said: "In the past, studies on walnuts have shown beneficial effects on many health outcomes like heart disease, diabetes and obesity.
"Our study was different because we focused on cognition, and in this controlled randomized trial we measured mood outcomes in males and females.
"There was a meaningful, 28 percent improvement of mood in young men.
"However we did not observe any improvement of mood in females. Why this is we do not know."
But the conclusion is a simple one, he added: "Eat more walnuts.
"They are not only good for your mood, but overall health as well. The recommended amount is one handful per day."
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