Who can resist those big puppy-dog eyes, floppy ears and fluffy tail? In a new study to be released by the ASPCA, 'physical appearance' is the top reason given for picking a particular puppy at an animal shelter.

With cats, it's a different story: 'Behavior with people' was what convinced most adopters to choose a particular adult cat.

The research, conducted by the animal-advocacy organization from January through May of 2011, involved five shelters across the country. About 1,500 adopters filled out questionnaires explaining how they knew the cat or dog was 'the one.' The findings are below. By understanding why people choose the pets they do, the ASPCA hopes to increase adoption rates and ensure adopters go home with a perfect match. It's particularly useful for shelter workers to know that appearance is often a deciding factor. They can then counsel adopters about behavior and other traits that might be overlooked.

'As an animal behaviorist, it was interesting to get inside the human animal's head,' says Emily Weiss, vice president of shelter research and development with the ASPCA.

The study supported findings from previousresearch showing that animals that approach the front of the cage when a visitor nears have a much greater chance of being placed in a new home. In the new study, many of the adopters who were asked, 'What did this pet do when you first met him/her?' specified a social interaction, such as an approach, a meow, a lick or even jumping on the visitor.

'That interaction is important for the human animal -- not just entertainment, but in choosing their next friend,' Dr. Weiss says.

Behavior with people: 77.9%

Physical appearance: 65.6%

Age: 63.9%

Age: 78.1%

Behavior with people: 69.3%

Physical appearance: 62.8%

Behavior with people: 78.3%

Physical appearance: 75.4%

Age: 65.6%

Physical appearance: 76.8%

Age: 74.8%

Behavior with people: 73.9%

Note: Respondents were able to pick multiple reasons.