how do dogs pick up on human emotions?

Question: how do dogs pick up on human emotions?
- Keywords:
  dog,
  emotion
Asked by anon-210517 to Tori, Titus, Stuart, Hannah, Gill on 14 Mar 2019.
- Gill Harrison answered on 14 Mar 2019:
  
  I don’t know how they do it, but I think they are amazing. My old dog used to change his behaviour depending on my mood.
- Stuart Higgins answered on 14 Mar 2019:
  
  There’s a report from 2016, that suggests that dogs use a mixture of looking at our faces and listening to the sounds we make. The scientists showed dogs photos of other happy/sad humans and then played a sound of the human being happy and sad at the same time. The dogs reacted more when the happy face was with the happy sound (or sad with sad), but not when there was a happy face with a sad sound. So the scientists concluded the dogs are smart enough to combine more than one piece of information to understand emotions!
  
  Journal article (really hard to read): https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2015.0883

Question: how do dogs pick up on human emotions?