Public transit riders in Toronto have been coming face-to-face with farm animals thanks to an ad campaign that asks "Why love one but eat the other?" The 1,000 poster-sized ads juxtapose pictures of pets such as cats and dogs with images of animals most people equate with food, namely chickens, pigs, and cows.

I am co-coordinator of this campaign, with friend and fellow animal advocate Kimberly Carroll. Together we conceived of and mounted the campaign to encourage thought and discussion, along with support from the Toronto Vegetarian Association and a whole community of people concerned about animals.

We expected strong reactions to the campaign and we got them. Some accused us of mounting a propaganda campaign, although all of the information we provide is factual. Some asserted that people need to eat animals to survive, although it has been medically proven they do not. The American Dietetics Association and the Dietitians of Canada have both endorsed a well-planned vegetarian diet as perfectly healthy. Some accused us of promoting false teachings according to the Bible or compared us to Muslim extremists, all in spite of the fact that the campaign makes no mention of religion.

One might ask why an ad on a subway is capable of eliciting such strong reactions. In my day job, I am a professor who specializes in behavioural finance, or how economic decisions are influenced by human psychology. Our ad campaign borrows a concept from this field of study: cognitive dissonance. When a random transit rider sees the images of cows, pigs, kittens, chicks, and dogs on our posters, his immediate impulse is likely to think "Aww, how cute! I love animals!" As his attention shifts lower down the poster, he is confronted with horrifying photographs of common practices on factory farms: for instance tail docking, beak cutting, and teeth clipping without anesthetic, cows with inflamed udders due to mastitis, pigs in stalls so small they cannot turn, and chickens stuffed so tightly into cages that they cannot spread their wings. Standard treatment of cows, pigs, and chickens in the Canadian agricultural industry would be illegal is applied to a cat or dog.

So the transit rider is confronted with two conflicting thoughts: "I love these cute animals!" and "When I eat animals, I am complicit in their cruel treatment!" This is cognitive dissonance -- simultaneously holding two conflicting thoughts in one's head. A common and sensible way for an individual to resolve cognitive dissonance is to change his actions. Extensive feedback on this campaign (in the form of video testimonials, emails, tweets, and Facebook comments) suggests many Canadians have decided to resolve their internal conflict by becoming vegetarian and vegan. We have scores of testimonials from people who have decided to stop participating in the exploitation of sentient beings after seeing the ads. Of course the campaign has its detractors as well, with some folks choosing to resolve their cognitive dissonance by adopting or maintaining the mistaken belief that cows, pigs, and chickens are somehow different from other animals and therefore unworthy of compassion. Our past experience with people transitioning to a vegetarian diet suggests that many of these folks will continue to ruminate over the conflicting thoughts and may eventually decide that changing their actions will lead to greater mental comfort than attempting to maintain a belief that flies in the face of the documented facts about animals and factory farming.

A different ad campaign might have invoked cognitive dissonance using a different set of facts, for instance by highlighting the adverse environmental or health consequences of eating animals. Our planet suffers greatly as a consequence of factory farming. Additionally, medical research strongly suggests that meat-eaters can face a significantly higher risk of life-threatening conditions such as heart disease, obesity, diabetes, stroke, and cancer.

Overall, this campaign demonstrates the role behavioural economics and human psychology can play in shaping important decisions that ultimately help people, animals, and the environment. And what about the fact that the campaign unexpectedly led to us being called a few names? We won't let that ruffle our feathers.

You can learn more about the subway ad campaign, which runs through to the beginning of 2012, by checking out the campaign web site www.BeVeg.ca or the campaign video.

Follow Lisa Kramer on Twitter: www.twitter.com/ProfLisaK