Experiment where groups of dogs were treated differently to find difference in affection
VSauce mentions this experiment in "Why Do We Kiss?" at 9m 24s in.


Topál, J., Miklósi, Á., Csányi, V., &amp; Dóka, A. (1998). Attachment behavior in dogs (Canis familiaris): a new application of Ainsworth's (1969) Strange Situation Test. Journal of comparative psychology, 112(3), 219.