First, he tore up his dog toys. Then shredded the furniture, clothes, schoolbooks — and, finally, any semblance of family unity. James, a chocolate-brown pointer mix, turned from adorable pet to problem child in a matter of weeks.

“The big bone of contention was that my mom and my sister thought that he was too smart to be treated like a dog; they thought he was a person and should be treated as such — well, spoiled,” said Danielle, a Florida woman who asked that her last name not be published to avoid more family pet strife. “The dog remains to this day, 10 years later, a source of contention and anger.”

Psychologists long ago confirmed what most pet owners feel in their bones: that for some people bonds with animals are every bit as strong as those with other humans. And less complicated, for sure; a dog’s devotion is without detectable irony, a lap cat’s purring without artifice (if not disapproval).

Yet the nature of individual human-pet relationships varies widely, and only now are scientists beginning to characterize those differences, and their impact on the family. Pets alter not only a family’s routines, after all, but also its hierarchy, its social rhythm, its web of relationships. Several new lines of research help explain why this overall effect can be so comforting in some families, and a source of tension in others. The answers have very little to do with the pet.