More and more Americans are waiting longer to get married — if they do so at all. The age of first marriage keeps rising, while rates of marriage are generally declining. The result is that many of us spend much of our adult lives living independently, and for a variety of reasons. Some find themselves solo due to marriages that go belly-up, while others enjoy a long single period before marrying. And some people figure out that being on their own suits them. In 2015, some 45 percent of Americans older than 18 were single — the highest rate ever.

But what does "single" even mean? Like a certain percentage of Americans, I will never marry (for a variety of personal and financial reasons), though I'm partnered with a wonderful man I enjoy sharing my life with. Am I single? Well, on paper, yes. But in reality, I enjoy most of the benefits and deal with the same frustrations of coupledom that married people do. And if you've heard about the long-term health benefits of coupling up, people who cohabitate but aren't married get them too.

Despite single people comprising a growing part of the population, psychologists and anthropologists don't know much about them. Why?

The 'problem' with studying single people

Woman happily living the single life Almost half of us spend our adult lives living independently, for a variety of reasons. (Photo: Antonio Guillem/Shutterstock)

Part of the issue is that it's usually married people who are studied, and single people tend to be ignored or used as "control" subjects, rather than being the focus of research.

Another problem is how "single" is defined, as in my personal example above. Also, someone can be single during some parts of their life and married for others. People who are currently single could have been married — or might marry next year. And why they are solo matters as well. It could be due to a health issue, because they have been widowed, or have gone through a divorce. People are single for different reasons, but in a research study, they likely would be put together in one group.

Differences between married and single people could be chalked up to fundamental differences in personality. For example, conservatives, who tend to be traditional, are more likely to be married than moderates or liberals, according to the Institute of Family Studies. And according to studies, married people tend to commit fewer crimes and have more self-control. But these findings could have little to do with marriage and everything to do with the fact that these people simply avoid risky behavior. The problem is we just don't know — and neither do the scientists who study these groups.

Are married people really healthier?

Still, many pop-culture extrapolations have been made about single people based on studies of the married — the most common is that marriage makes people healthier (and therefore singletons are sicker). This could be because someone who marries is more likely to be healthy in the first place, or it could have little to do with the individual: In the United States, where until recently healthcare was not available to all, those who married could get health insurance through a spouse. We know access to healthcare via insurance is proven to increase overall health, so marriage might supply some people with insurance and therefore better health, with the actual tying-of-the-knot having nothing to do with it.

How about those mental health benefits? They might be short-lived positive bumps. A 2012 study that followed 2,700 people for six years found that while married people were less depressed and had higher levels of self-reported well-being, those feelings faded over time.

As the report's abstract stated: "The effects of marriage and cohabitation are found to be similar across a range of measures tapping psychological well-being, health, and social ties. Where there are statistically significant differences, marriage is not always more advantageous. Overall, differences tend to be small and appear to dissipate over time, even when the greater instability of cohabitation is taken into account."

While the idea that marriage is somehow better than being single is baked into our culture, there's not a whole lot of evidence that it's true. Nonetheless, the federal government spends millions a year on promoting marriage — to the tune of almost a billion dollars since President George W. Bush funded the Healthy Marriage Initiative in 2004. The results have been negligible: All that spending hasn't changed marriage rates or divorce rates.

Maybe the assumption that being single is a terrible fate is the idea that needs to change, and that however you end up, choosing marriage (if that's what you want) will benefit from longer timelines and careful consideration. In her new book, "All the Single Ladies: Unmarried Women and the Rise of an Independent Nation," Rebecca Traister writes, “I wound up happily married because I lived in an era in which I could be happily single.”

Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.

Why scientists don't do a good job of studying single people
People who are single are a growing part of the population, so why don't we know more about them?