There are a slew of reasons why couples may be living apart. They may attend different colleges or graduate schools, one spouse may be in a nursing home, or one spouse may be incarcerated.

But a big — and growing — divider of couples is work, says Danielle Lindemann, a sociologist at Lehigh University and the author of a book on commuter spouses.

New job opportunities often pull people away from their partners and families for years at a time, especially as employment opportunities centralized in a few superstar cities.

By the numbers: Around 40% of the 97 commuter couples Lindemann interviewed for her book had minor children when they lived apart.