Apsychologist investigating how teens are influenced by their peers is most likely to be in the subfield of

That's a good question!

The sub-field that you speak of is called social psychology. Social psychology is the part of psychology that deals with social interaction.  It's also based on the effects that socializing has on a individual.

It's a simple part of psychology and it's fun to learn about. 

If it's a smaller child or infant, place one on the middle of the chest and on the child's back. (if it is a bigger child with enough room for two, place one on the upper right/your left of the chest, and one on the lower left/your right) the pads should never touch each other.