A second problem is political responsiveness: The policies of large nations can be slow to change, even if change is needed and desired. In a book published last year, the sociologists John Campbell and John Hall compared the reactions to the 2007-2008 financial crisis in Denmark, Ireland and Switzerland. These three small countries didn’t cause the crisis; a homegrown Irish housing bubble notwithstanding, the shock wave they dealt with came from America. But though the countries were economically vulnerable, Mr. Campbell and Mr. Hall observed, this vulnerability fostered unexpected resilience and creativity, generating in each nation “a sense of solidarity or ‘we-ness’” that brought together politicians, regulators and bankers eager to do whatever was necessary to calm markets.

With the United States lacking the same sense of shared fate and vulnerability, American policymakers could organize only a tepid response, which helps explain why the recovery here was so slow. This theory sheds light as well on developments in environmental and social welfare policy, where it is increasingly common to find a complacent America lagging behind its smaller, more innovative peers.

Finally, largeness can take a toll on citizen trust. The presence of a wide variety of social groups and cultures is the primary reason for this. Nearly all scholars who study country size recognize, as Madison did, that large nations are more socially heterogeneous, whether because they represent an amalgamation of different regions, each with its own ethnolinguistic, religious or cultural heritage; or because their economic vitality encourages immigration; or because population size and geographic spread promote the growth of distinctive subcultures; or because they have more differentiated class structures.

It isn’t inevitable that a large amount of social variation would undermine trust. Well-governed societies like Canada address the issue by stitching diversity and multiculturalism into their national identities. Yet in the absence of cultural and institutional supports, heterogeneity and trust are frequently in tension, as different ways of life give rise to suspicion and animosity. Without at least a veneer of trust among diverse social groups, politics spirals downward.

The challenges of American largeness are here to stay. The task now is for individuals, civic organizations and institutions to commit themselves to building stronger communities and a renewed sense of shared responsibility and trust among different groups. Within the constraints of our nation’s size, we can create conditions for as much democracy as possible.