There are roughly (or, if you prefer, ruffly) a billion dogs on the planet. Although dogs and humans have been existing alongside one another for tens of thousands of years, experts say 85 percent of the world’s dogs are feral. Roaming streets and villages, these canines aren’t domesticated pets but still hang out around human habitats.

When pups are between two and three months old, their mothers will abandon them for any number of reasons. With no mother to watch out for them, infant mortality of pups under one year skyrockets to around 90 percent. So, only about 10 percent of motherless, homeless pups survive.

Picture-Perfect Pups

The participants were asked to judge the puppies’ “attractiveness” rather than “cuteness,” because “we wanted to keep it neutral,” Wynne tells National Geographic. “We didn’t want to nudge people toward infantile features.”

The researchers thought that people would find pups most attractive between two and three months old. That’s around weaning age, the time when the puppies are abandoned by their mothers and need another caretaker to survive. They were right. (Related: “Why Are Dogs So Friendly? Science Finally Has an Answer”)

The pups peaked at different ages, but they were all ranked adorable between six to eight weeks. Jack Russell terriers peaked at 7.7 weeks, cane corsos at 6.3 weeks, and white shepherds at 8.3 weeks. All three breeds showed an uptick in cuteness when they reached 30 weeks old, but it’s unclear why.

Harold Herzog, an expert on human-animal interactions at Western Carolina University, wrote about the study on his research blog. He says the study is “brilliant” but could be tweaked.

Brain Games for Old Dogs Could Improve Their Mental Health Scientists are testing if they can improve cognitive function in older canines.

“I think it would be great for the study to be replicated using actual photographs of the same dogs as they aged through that critical period,” Herzog says. His hunch is the replication would support the results. Wynne says that some of the photos are of the same dogs at different ages, but most of them are different dogs.

Herzog adds another study could include wolves as a control. In contrast to dogs, both wolf parents will care for their offspring two years after they’re born, so that species likely wouldn’t show “the same trajectory of cuteness.”

Wynne says that going forward, researchers could show participants 20- to 30-second videos of puppies to see if there’s something in their movements that attracts people. The study was inspired by a research trip Wynne took to the Bahamas, where there are countless stray dogs. (Related: “Welcome to the Land of a Thousand Stray Dogs”)

“If this has any meaning in dogs’ and people’s lives, then it would be the actual physical moving animal that people would be seeing,” Wynne says.

Dog-Eat-Dog World

Since newly abandoned pups are competing with each other for human heartstrings, evolution says they should be most adorable around six and 11 weeks. This is around the time they’re weaned and let go of by their mothers. (Related: “Does Your Dog Prefer You Over Anyone Else? It’s Complicated”)

“What Clive [Wynne] is arguing is, unless you’re cute, you’re going to die,” Herzog says.

There are a few characteristics that humans find particularly adorable across species: big, forward-facing eyes, floppy and unstable limbs, and a soft, rounded body shape. We’re also keen to squeal when animals have large heads in comparison to their bodies, and this reaction goes back to evolution.

Called kinderschema, these qualities are also apparent in human babies and necessary for their survival. The characteristics activate the decision-making part of the brain to encourage you to protect and nurture the baby. At the same time, the brain’s pleasure center releases dopamine. With these two reactions, your brain makes you want to protect the baby and rewards you for doing so. With your protection, the baby can survive. (Related: “When We See Something Cute, Why Do We Want to Squeeze It?”)