The person you marry makes a huge difference--not just professionally but also personally. Science proves it's true. I know from experience it's true. And so does Nascar driver Dale Earnhardt. Jr.

I'm not saying I know Dale Jr. well. At all. We've only spoken three times. Once for about 20 minutes last May in Mooresville, North Carolina. Once for about 20 seconds when I ran into him at Homestead while I was doing a piece on Alan Gustafson, the crew chief for Chase Elliott, one of his teammates at Hendrick Motorsports. And then on Tuesday, when we talked about his podcast, his recovery from concussion and return to racing, and how he maintains focus under pressure.

It's only one data point, but on Tuesday I saw a different Dale. By the time I spoke with him he had already appeared on the Today show and the Dan Patrick Show, and conducted two phone interviews on the drive over. Then he did an interview with me, another with a local television station, a phone-in for Jim Rome, two other phone interviews...and then left for two more appearances.

Imagine being "on" for a full day, bringing energy and enthusiasm to each and every interview...even if you've been asked the same questions dozens of times.

I couldn't do it. He can.

But that's not all. I'm fairly good at reading people, and on Tuesday, Dale also seemed more relaxed, casual, and in the moment. Outside the interview we talked about sushi. We talked about what it's like to be in the booth for a national television broadcast. We talked about how he picked out his wife's engagement ring (a risky move, since she didn't see it beforehand, but he was smart enough to do a little quiet sleuthing.)

In the middle of the whirlwind that is his life, he seemed to be exactly where he wants to be. Isn't that what we all want? (Isn't that a great way to define success?)

Lots of factors go into success. Your mindset plays a huge role. Developing greater determination, willpower, and grit (because they can be developed) is key. Being more likable makes a huge difference. (And, yes, you can develop that quality too.)

While all those are important factors in the success of your business or career--and therefore your earning power--here's one factor that is often ignored:

The person you marry.

Researchers at Washington University in St. Louis found that people with relatively prudent and reliable partners tend to perform better at work, earning more promotions, making more money, and feeling more satisfied with their jobs.

That's true for men and women: "Partner conscientiousness" predicted future job satisfaction, income, and likelihood of promotion (even after factoring in the participants' level of conscientiousness).

According to the researchers, "conscientious" partners perform more household tasks, exhibit more pragmatic behaviors that their spouses are likely to emulate, and promote a more satisfying home life, all of which enables their spouse to focus more on work.

As one researcher said, "These results demonstrate that the dispositional characteristics of the person one marries influence important aspects of one's professional life." In non-research-speak, a good partner both sets a good example and makes it possible for you to be a better you.

Dale's wife, Amy, accompanied him Tuesday. (She's delightful.) And I think--again, taking a strong position based on limited data--that she's made a huge difference for him, not just personally, but also professionally.

Of course a few of those data points are significant. He says she is a grounding force in his life. He says she played a huge role in motivating him to work hard at his recovery from concussion and return to not just racing, but health.

And here's the thing: He makes his living as a driver, but he spends a significant percentage of his time representing his team and his sport. That's the job. He's paid to win...but he's also paid to talk, and the better he represents his team, his sponsors, and his sport, the better he is at his job. (A driver who performs poorly on the track will never overcome that by being personable and engaging off it, but a driver who can win on the track and is also great off the track can take his or her career to the next level.)

I know what you're thinking: "Why use Dale as an example of greater success? Isn't he already incredibly successful?" Absolutely--and that's the point. Your spouse, your partner, can help you be more matter how successful you may already be.

As Jim Rohn says, we are the average of the five people we spend the most time with--and that's particularly true where our significant others are concerned. Bad habits rub off. Poor tendencies rub off. We all know that. But good habits and good tendencies rub off too.

Of course, I don't recommend you choose your significant other solely on the basis of criteria like conscientiousness or prudence. As the researchers say, "Marrying a conscientious partner could at first sound like a recipe for a rigid and lackluster lifestyle." Nor am I suggesting you end your relationship if you feel your partner is lacking in those areas.

But it does appear that marrying the right person is a key ingredient in the recipe for a better and more rewarding career.

So instead of expecting your partner to change, think about what you can do to be more supportive of your significant other. After all, the best way to lead is by example.

In time, you may find that you and your significant other make a mutually supportive, outstanding team, whatever your individual and joint goals may be.

And you may also find yourself exactly where you want to be--which, upon further reflection, is the perfect definition of success.

Because that means you're happy.