Laughter is social because all people do it.
Dr. Dunbar used a variety of evidence from the study, such as .
While each of the types of evidence from to proved both quantitatively and qualitatively that , the strongest piece of evidence was .
His results were .

One drawback of voluntary trade is that it a.requires two groups to use a common language b.can make one group dependent upon another for necessary goods c.doesn't occur unless there is a shortage of goods for at least one group d.forces groups to rely on themselves to produce all the goods they need