Hugs are one of the first forms of contact and affection that humans experience. Not only are hugs a common way to provide comfort, support, or show affection, they have also been shown to have health benefits. Hugs can lower blood pressure, improve oxytocin levels (the hormone that makes you happy), lower cortisol levels (the hormone that makes you stressed), and improve your immune system. As roboticists who study human interaction, we are therefore naturally interested in one day having robots hug humans as seamlessly as people hug each other. The HuggieBot project's purpose was to evaluate human responses to different robot physical characteristics and hugging behaviors. Specifically, we aimed to test the hypothesis that a soft, warm, touch-sensitive PR2 humanoid robot can provide humans with satisfying hugs by matching both their hugging pressure and their hugging duration.
Thirty relatively young participants with rather technical backgrounds experienced and evaluated twelve hugs with the robot, divided into three randomly ordered trials that focused on physical robot characteristics (single factor, three levels) and nine randomly ordered trials with low, medium, and high hug pressure and duration (two factors, three levels each). Analysis of the results showed that participants significantly preferred soft, warm hugs over hard, cold hugs. Furthermore, users preferred hugs that physically squeeze them and then release immediately, when they are ready for the hug to end. Taking part in the experiment also significantly increased positive user opinions of robots and robot use.