In the years following the American Revolution, not everyone expected New York to emerge as America’s preeminent city. Philadelphia had hosted the Constitutional Convention. Boston was a crucial manufacturing hub. And even among the cities crowding the lower Hudson (then called the North River), some believed that those on the western shore were more likely to flourish. A figure no less prominent than Alexander Hamilton predicted that Jersey City would eventually become the “metropolis of the world.”

There was good reason. New Jersey boasted all the advantages of Manhattan without many of its liabilities. Both offered access to the same harbor, and then to the Atlantic. Both were situated along a navigable river that would eventually connect to the Erie Canal. But while Manhattan real estate was relatively scarce, Jersey’s expanse seemed limitless. Just as important, goods manufactured in the Garden State were already on the mainland, and could be shipped to points south and west without having to traverse an extra body of water.

Nearly two centuries later, Jane Jacobs offered a novel explanation for New York’s triumph. In The Economy of Cities, published in 1969, she argued that the elements most scholars cited when trying to explain metropolitan success—access to natural resources, for example—obscured one monumentally important factor: the random collision of ideas.

Several years earlier, the famed journalist Arthur Koestler had published The Act of Creation, a book which argued that “the creative act is not an act of creation in the sense of the Old Testament. It does not create something out of nothing; it uncovers, selects, re-shuffles, combines, synthesizes already existing facts, ideas, faculties, skills.”

Jacobs applied Koestler’s insight to the urban landscape. Successful cities, she contended, managed to overlay different industries, communities and experiences. Growth is born from innovation, and innovation emerges only when concepts can jump easily from one field to another. Because those living, working and playing in New York they were trapped between the East and Hudson Rivers and couldn’t so easily escape one another, intellectual cross-fertilization was almost inevitable. In other words, Manhattan’s tighter quarters were instrumental to the Big Apple’s success.

Since then, the Koestler/Jacobs theory has become a staple of popular business literature. Pick up a recent edition of the Harvard Business Review and you’ll be hard-pressed to avoid stumbling on a story extolling the virtues of “breaking out of your silo.” But those looking to spark innovation on a wider scale rarely think about social architecture even as America sleepwalks through an entrepreneurial crisis. Statistical measures of new business creation have been halved since the 1970s. As one economist testified last year before the Senate Small Business Committee: "Millennials are on track to be the least entrepreneurial generation in recent history.”

Beyond asking whether investing in better schools or more research and development, it’s time we entertain another possibility: American innovation may be suffering from the fact that Americans today have less exposure to ideas outside the realm of their own experience.

What would it take to get ideas bouncing back-and-forth? To begin, ordinary people from disparate parts of society need to know one another. Do they?

In 2000, Robert Putnam’s Bowling Alone laid out reams of evidence suggesting that Americans were becoming increasingly isolated from one another. But critics countered that digital technology (and now social media) put billions of people mere clicks away from an exchange of ideas; globalization, they argued, actually brings strangers together. What’s clear now is that that argument missed the point. Whether or not we have more or less “social capital” in the aggregate, we’re inarguably choosing to invest our time and energy today in different sorts of relationships. And as Jacobs would point out, who we know has a profound effect on what we think.

America’s social architecture has undergone a subtle but profound transformation over the last several decades. Data from the General Social Survey suggests that Americans have maintained roughly the same amount of time with their most intimate acquaintances (the handful with whom we exchange text messages) and their more ephemeral contacts (the elementary school pals we friend on Facebook). From 1974 to 2014, the percentage who reported spending a “social evening” with a relative more than once a month fell just slightly from 58 percent to 55 percent. The share spending more than one evening with a friend who lived outside their neighborhood actually grew from 40 percent to 42 percent. But the percentage reporting a social evening spent with someone who lived near them plummeted from 44 percent to 32 percent, suggesting that we’ve broadly abandoned the middling contacts that were once a staple of the American life. For better or worse, we’ve become strangers to the neighbors, Rotarians, bridge partners and bowling-league teammates who would once have been more familiar.

Which brings us back to the roots of intellectual cross-fertilization; after all, not every category of relationship brings people with different points of view into close proximity. Coining the phrase, “the strength of weak ties,” Stanford sociologist Mark Granovetter argued decades ago that more intimate contacts rarely proffered new concepts, if only because we know what our closest acquaintances think already. That might augur for more ferment today if only because weak ties have proliferated through the digital revolution. But a distinction exists between familiar and entirely ephemeral connections. A true exchange of ideas demands more depth than what is often on offer in the comments section of a Facebook post.

We should be so lucky to find simple ways to stir the pot. If, as some suggest, Americans lived in denser environments, used mass transit, and sent their children to public schools, they might be more prone to encounter new thinking. But as Bill Bishop argued in The Big Sort, even our neighborhoods have become more monolithic because we’re prone to settle among people who share our sensibilities. And it’s not just that. Look at all the people walking their dogs and riding the subway with earbuds that make them deaf to the world. On the whole, Americans have become increasingly private even in public spaces.

Nearly two decades ago, the University of Chicago’s Ronald Burt released the findings of a study he’d done of Raytheon, a defense contractor that boasted state-of-the-art physical plants, talented engineers, access to many of the world’s greatest academic institutions and much more. Trying to uncover the roots of new thinking, Burt concluded that the Raytheon employees most likely to have good ideas weren’t those entrenched most deeply in any one field of expertise or another. Rather, the unlikely stars were those few who “spanned structural holes.” In other words, the most intellectually disruptive employees were those with their hands in the work of more than one department.

What would it take to compel Americans to span more structural holes in their everyday lives? I’ve yet to stumble on a wholly satisfactory answer. There’s great promise in programs like AmeriCorps, which places Americans from wildly different backgrounds into eighteen-month service projects. But maybe more hopeful is the progress educators have made imbuing young students with additional “grit.” The most pervasive barrier preventing individuals from exploring new ideas often centers on a fear that substantive encounters outside a familiar bubble will be awkward, dangerous, or uncomfortable. Someone might discover that an interesting stranger voted for the other presidential candidate—or harbors some sort of ugly prejudice.

Exposure to new ideas often requires the ability to maintain your equanimity in the face of disagreement. A grittier America would be less likely to lash out or turn away in disgust.

We can hardly fault Alexander Hamilton for being born more than 200 years before Jacobs published The Economy of Cities . But we should heed the warning of his mistaken prediction that Jersey City would become today’s Manhattan. We’ve finally solved the mystery of how good ideas emerge. Our challenge now is to is compensate for the sociological shift that has begun to temper the hot stew of intellectual cross-fertilization.

Marc J. Dunkelman is a fellow both at Brown University’s Watson Institute for International and Public Affairs and NYU’s Marron Institute of Urban Management. His first book, The Vanishing Neighbor: The Transformation of American Community, was published by W.W. Norton.