If you're 30 years old and feel less happy than you did when you were 20, science says you're not alone. If you're 40 years old and feel less happy than when you were 30, science says you're also not alone.

And if you're 47.2 years old and feel less happy than you did when you were 40, recent research says you're definitely not alone.

Why? Research conducted by Dartmouth professor David Blanchflower on hundreds of thousands of people in 132 countries shows that people around the world experience an inverted, U-shaped "happiness curve."

Starting at age 18, your happiness level begins to decrease, reaching peak unhappiness at 47.2 in developed countries and 48.2 in developing countries.

The good news is that happiness levels then gradually increase.

The bad news is you're unlikely to feel as happy as you did when you were 18 until you're in your mid-60s. (Eek.)

Money isn't really a factor. According to Blanchflower, "The curve's trajectory holds true in countries where the median wage is high and where it is not," he says. 

Nor are a few other socioeconomic drivers. After studying fifteen measures of unhappiness -- anxiety, despair, sleeplessness, sadness, depression, fatigue, tension, strain, etc. -- controlling for factors like employment status and education stil resulted in the inverted U-shape happiness curve. 

Here's the chart for the U.S. (Again, the curve is inverted: Rising lines indicate increasing levels of unhappiness, falling decreasing unhappiness.)

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In short, as Blanchflower says, "The happiness curve is everywhere."

Blanchflower has a few theories to explain the phenomenon. One is expectations. When we're twenty, the world is our oyster. We'll do great things. We'll achieve all our dreams.

But things don't usually work out that way -- and it takes a long time for people to gain the wisdom, maturity, or perspective required to, as Blanchflower puts it, "quell their infeasible aspirations." (Evidently it takes us until we're 47.2 years old.)

Another theory revolves around comparisons. Sometimes comparisons can be useful... but where your sense of satisfaction and fulfillment are concerned, they're definitely not. Branchflower speculates that people in their 20s, 30s, and early 40s are more likely to compare themselves to others -- and find themselves wanting. 

Do that, and no matter how successful -- or well traveled, or social, or educated, or whatever your FOMO measure might be -- and there will always be someone who seems more successful. There will always be someone better, or smarter, or wealthier, or (seemingly) happier.

What You Can't Control...

Regardless of the underlying causes, Branchflower's research provides a scientific basis for experiencing a mid-life crisis. 

Feeling unhappy when we realize we won't achieve all of our goals, dreams, and aspirations is something we can't control -- and hits us hardest at age 47.2

But then we gradually accept that failing to achieve every dream we once held dear isn't the end of the world, and slowly start to focus more on enjoying what we do have than what we don't.

According to Branchflower, that phenomenon is also something we don't necessarily control: Other species, most notably great apes, experience a similar happiness curve. 

And What You Can Control

While the happiness curve is a natural phenomenon, one you can't totally avoid experiencing, you can reduce its impact on your life by taking a few steps science proves will make you feel happier:

Make a few close friends.

The Framingham Heart Study shows that geographically close friends have the greatest effect on your happiness.

Virtual friends are great. Distant friends are great. But the closer your friends are to where you live, the better. (Plus, the positive effect of close relationships on your lifespan is double what you get from exercising. And just as powerful as not smoking.)

Start comparing yourself to yourself.

Look hard enough and you can always find someone who seems happier, especially if your yardstick is other people's carefully curated social media lives. Look hard enough and someone else will always seem more successful, more fulfilled... more something.

Instead, compare yourself to yourself.  

Pick a goal, and measure yourself against that goal -- because that is the only comparison that matters.

Help other people.

While giving helps other people, giving also helps you -- in fact, research shows that providing social support may be more beneficial than receiving it.

Helping someone in need is not only fulfilling, it also reminds us of how comparatively fortunate we are -- which serves as a great reminder of how thankful we should be for what we already have.

Which leads to one more thing you can do to minimize the effect of the haappiness curve.

Express gratitude -- every day.

In one study, couples who expressed gratitude in their interactions with each other experienced increased relationship connection and satisfaction the next day -- and that applied both to the person expressing thankfulness and (no big surprise) the person receiving it.

(In fact, the study's authors say gratitude is like a "booster shot" for relationships.)

Of course, the same is true at work. Express gratitude for an employee's hard work and both of you will feel better about yourselves.

Another easy method of fostering thankfulness is to write down a few things you are grateful for every night. At least one study shows that people who wrote down five things they were thankful for once a week were 25 percent happier after 10 weeks

The happiness curve results, in part, from dwelling on what you don't have.

Happy people focus on what they do have.

While it's motivating to want more -- in fact, it's healthy to strive for more -- taking the time to express gratitude for what you already have will make you feel a lot happier.

And will help remind you that although you may still have huge dreams... you have already accomplished a lot.

Give yourself permission to feel good about it.