As one puppy tugged her shoelace, another went for her long hair, and yet another tugged on her pants leg, I watched my sister’s reaction. It suddenly occurred to me. “You’ve never played with an entire litter at once, have you?” I asked.

She hadn’t. In fact, her dogs were rescued as adolescents, so she’s never lived with a young puppy either. These Labs were seven weeks old, the perfect age: cute, cuddly, and playful, but still too small to be pushy or overwhelming en masse (there were seven of them, after all).

Seven weeks is also about the age that most litters are split up, each puppy being sent off to his or her new home. Some breeders (and puppy mills) try to place puppies even younger than seven weeks; responsible, caring breeders will wait at least eight weeks — maybe longer.

The puppies need that time together.

A study published in the Veterinary Record in October 2011 shows that puppies who were removed from their litters too early were far more likely to exhibit problem behaviors later in life than puppies who stayed with their sibs for 60 days or longer. The behaviors included destructiveness, excessive barking, fearfulness on walks, reactivity to noises, toy possessiveness, food possessiveness, and attention-seeking. A much larger proportion of early-separated dogs demonstrated those behaviors.

The study was based on survey responses of 140 dog owners in Italy. Their dogs were between 18 months and seven years old. Half had been adopted at age 30–40 days and half at 60 days. Half came from pet stores and the rest from friends or relatives of the owner or from a breeder; none had been adopted from shelters or had been obviously traumatized.

Dogs who had been separated early from their litters and purchased from a pet store showed the greatest tendency toward toy possessiveness, fearfulness on walks, attention-seeking, stranger aversion, excessive barking, destructiveness, and play biting. Thus, the authors concluded, the combination of early separation and temporary housing at a pet store is particularly inhibiting to a puppy’s social development.

Dog experts have long known about sensitive periods in puppy development and the importance of puppies learning, through play with their littermates, to interact appropriately with other dogs. A puppy’s experiences during this sensitive period, that starts at about 2.5-3 weeks of age and ends at 12 to 14 weeks, a puppy’s experiences can actually modify his brain, leaving lasting effects on his temperament and behavior.

In plain English, this study suggests that taking a puppy away from Mom and sibs too early could affect his ability to adjust to new environments and social relationships — for the rest of his life. It also suggests that early separation makes the puppy more likely to develop behavior problems.

Read more about this study here.