Fad diets aside, we all know the basic formula for greater physical health -- eat less junk and exercise more. The same can be said for greater happiness. Sure, mental health is hugely complex, but the research on how to promote basic, day-to-day well-being couldn't be clearer -- just cultivate gratitude.

"Something as simple as writing down three things you're grateful for every day for 21 days in a row significantly increases your level of optimism, and it holds for the next six months. The research is amazing," Harvard researcher and author Shawn Achor has told Inc.com. Other studies show gratitude increases willpower, helps keep you calm, and can even boost employee morale.

All of which is both interesting and useful, but it begs the question: Why is simply paying attention to the good things in your life so powerful? A new brain imaging study investigated this question with fascinating results.

This is your brain on gratitude.

For the study, a team of researchers out of Indiana University led by Prathik Kini recruited 43 subjects suffering from anxiety or depression. Half of this group were assigned a simple gratitude exercise -- writing letters of thanks to people in their lives -- and three months later all 43 underwent brain scans.

During these brain scans the subjects participated in a gratitude task in which they were told a benefactor had given them a sum of money and were asked whether they'd like to donate a portion of the funds to charity as an expression of their gratitude. Those who gave away money showed a particular pattern of activity in their brains, but that wasn't the most interesting part of the findings.

What was? "The participants who'd completed the gratitude task months earlier not only reported feeling more gratefulness two weeks after the task than members of the control group, but also, months later, showed more gratitude-related brain activity in the scanner. The researchers described these 'profound' and 'long-lasting' neural effects as 'particularly noteworthy,'" psychology writer Christian Jarrett explains on the Science of Us blog.

The result is interesting for neuroscientists but it's also potentially useful for the rest of us. It "suggests that the more practice you give your brain at feeling and expressing gratitude, the more it adapts to this mindset -- you could even think of your brain as having a sort of gratitude 'muscle' that can be exercised and strengthened... the more of an effort you make to feel gratitude one day, the more the feeling will come to you spontaneously in the future."

In short, practicing gratitude seems to kick off a healthful, self-perpetuating cycle in your brain -- counting your blessing now makes it easier to notice and count them later. And the more good you see in your life, the happier and more successful you're likely to be.

Or, as Jarrett sums up the research: "The more you practice gratitude, the more attuned you are to it and the more you can enjoy its psychological benefits."