If you don't have rich parents, the difference between riding in coach and taking private jets might depend on where you grow up.That's according to a study published in the scientific journal Nature in two parts on Monday. Researchers who studied 21 billion Facebook friendships found that children have a higher chance of eventually climbing social and professional ladders if they're friends with kids in higher-income families. And such socioeconomic mingling is more common in some parts of the country than others, the study noted.The study's authors measured the "social connectedness" — or meaningful interaction between rich and poor residents — in counties across America, and determined that "if children with low-income parents were to grow up in counties with economic connectedness comparable to that of the average child with high-income parents, their incomes in adulthood would increase by 20% on average."The study also noted that the value of social connectedness "is equivalent to the difference in average outcomes between a child who grows up in a family that makes $47,000 a year instead of $27,000 a year."Out of the country's 200 largest counties, these are the study's top 11 cities where such social connectedness thrives:San FranciscoUtah County, UtahLoudoun, VirginiaSnohomish, WashingtonNorfolk, MassachusettsFairfax County, VirginiaSan Mateo, CaliforniaWaukesha, WisconsinSanta Clara, CaliforniaDavis, UtahHonoluluSan Francisco leads the pack: Residents have roughly an 80% chance of being exposed to high-income peers, and poorer people are about 6% more likely to befriend someone in a higher income household, the study said.The reason has nothing to do with admiring your friends' fancy cars or designer bags. Rather, wealthy parents often pour resources into teaching their children how to network, apply for jobs and acquire a bevy of professional skills — and the children tend to share their learnings with the people around them, according to the study.Matthew Jackson, an economics professor at Stanford and one of the study's authors, says several factors could explain why the phenomenon is more common in some cities than others, ranging from the size of local high schools to the particular attitudes of specific communities.One of the largest factors is the average income of a city's residents, Jackson says: The median household income in San Francisco is $119,000 per year, according to the U.S. Census. By comparison, Cameron County, Texas — one of the country's worst cities for social connectedness, the study said — has a median household income of $41,200 per year."Often, the areas that are really [socially connected] are mostly rich people," Jackson tells CNBC Make It. "I think that's one of the challenges: How do you build these connections in areas where you're predominately dealing with poor people?"Jackson also says the financial segregation of richer and poorer communities comes into play. If people of varying levels of income don't live in the same neighborhoods, they'll interact less.This research isn't a closed book. For example, Jackson says he's still trying to figure out why Minneapolis appears to be much more socially connected than Indianapolis, despite similar demographics across the two Midwestern cities."It's hard to know what exactly is responsible for [that difference] and how much of it is dependent on culture," Jackson says. "I think there's a lot more we'd like to know about what exactly can foster these cross-class ties."Jackson says he hopes that more data can help bring more people out of poverty."This data can help school administrators and community leaders understand why people aren't connecting, and hopefully it will influence policies," he says. "But this is an ongoing project. Now that the scale of this data and the richness of it is available to lots of researchers, we hopefully can start answering all kinds of questions."Sign up now: Get smarter about your money and career with our weekly newsletterDon't miss:Here’s how much money it takes to be considered wealthy in 12 major U.S. citiesBye bye, San Francisco: The top 7 U.S. cities homebuyers are seeking to leave
These 11 U.S. cities give you the best chance to earn more money than your parents—and none are New York or LA.