Recent studies have found that people are more likely to deny or minimize animal minds when they think of the animals as food, or when they expect to eat meat soon:
[M]eat eaters were asked to think about cows and sheep. Some of them thought about these animals living an idyllic life on a farm. Others thought specifically about these animals growing up on a farm and then being killed for food. Later, they also rated the mental abilities of the animals. When people thought about the animals as food, their ratings of the mental abilities of the animals were lower than when they thought about the animals living on a farm.This abstract of the studies theorizes that we deny animal minds in order to reduce our own cognitive dissonance. The dissonance can result from simultaneously wanting to eat meat, yet not wanting to harm beings that have minds.
It isn't just thinking about animals being used for food, though. In one final study, all of the participants had to write about the process of raising and butchering animals for food. All of the participants thought they were going to do a food sampling task after writing the essay. Half of the participants were told they would be eating fruit during the food sampling, while others were told they would be eating beef and lamb. Finally, participants rated the mental abilities of cows and sheep. The group that was about to eat meat gave much lower ratings of the mental abilities of cows and sheep than the group that was about to eat fruit.