It&#x27;s difficult to find any research in this area.  What I could find generally says that the advances made in earlier years are more significant than anything recent.<p><a href="http:&#x2F;&#x2F;www.cepr.org&#x2F;sites&#x2F;default&#x2F;files&#x2F;policy_insights&#x2F;PolicyInsight63.pdf" rel="nofollow">http:&#x2F;&#x2F;www.cepr.org&#x2F;sites&#x2F;default&#x2F;files&#x2F;policy_insights&#x2F;Poli...</a>