Now, a team of psychologists at the University of Wisconsin think they have a way to help reverse the trend.
It's something they call the "kindness curriculum."
Young kids start by learning the alphabet, colors and numbers - but is kindness a skill that can be taught?
Neuroscientist Richard Davidson is the director of the Center for Healthy Minds at the University of Wisconsin-Madison.
"It's fundamentally no different than learning how to play the violin or learning to do sports," he said.
Davidson developed the mindfulness based kindness curriculum for preschoolers to help them pay closer attention to their emotions.
"Part of the curriculum involves being able to tune into sensations in the body and learn to identify them and respond to them in an appropriate way," he explained.
One of the techniques he uses in class that parents can also use with kids at home, is belly breathing.
There's research that supports the fact that belly breathing can have calming effects, reduce muscle tension and increase focus.
While there are several practices of this technique, one common one says that parents and kids should first find a comfortable place to sit. Then, following the same directions, have your child place on hand on their chest and other on their belly.
Tell them to imagine they are blowing up a balloon in their belly. Breathe deeply through the nose, fully inflate the belly, then exhale through the mouth. You can even make a long "whoosh" noise when you exhale so they see and hear how it's done.
Davidson tested the kindness curriculum on a group of preschoolers.
"We found that kids who went through the kindness curriculum behaved more altruistically," he said.
He also found that the kids in the kindness curriculum had a better attention span, better grades, and showed a higher level of social competence.
Instructor Emily Golliher uses the kindness curriculum in her elementary school and feels it's vital for child development.
"If we can spend time and teach students how to be kind to themselves and kind to others that is just going to have a ripple effect across the school environment," she said.
Experts say besides the belly breathing technique, there are simple kindness strategies parents can try at home.
For example, really give kids your full attention instead of talking to them while keeping your eyes on your phone or other device.
Kneel down to speak with them and ask them questions about what they're telling you.
Kids are very observant and notice subtle nuances, so don't just pretend to listen - they will pick up on it!
And try to cultivate a moment of stillness every day by taking a few in and out breaths together.
The other tip? Psychologist say kids often model the behavior they see in adults.
So be open with them when you're having a tough day. By calmly explaining how to you feel, it may help them deal with stress and a bad day, too.
The mindfulness-based kindness curriculum is free to download from the Center of Healthy Minds website and it's available in both English and Spanish.