The study compared children enrolled versus forced to delay entrance according to an arbitrary date.[1] Socioeconomic status has nothing to do with it.<p>Further, the study explicitly claims causal relationships, rather than merely correlative. I&#x27;m guessing as a result of the kind of fancy-pants regression analysis they used. [2]<p>[1] &quot;children who were born a certain number of days before the Dec. 31 enrollment cut-off and those born after the cut-off&quot; <a href="http:&#x2F;&#x2F;www.nber.org&#x2F;papers&#x2F;w21610" rel="nofollow">http:&#x2F;&#x2F;www.nber.org&#x2F;papers&#x2F;w21610</a><p>[2] &quot;We estimate the causal effects of delayed school enrollment using a &quot;fuzzy&quot; regression-discontinuity design&quot; <a href="http:&#x2F;&#x2F;www.nber.org&#x2F;papers&#x2F;w21610" rel="nofollow">http:&#x2F;&#x2F;www.nber.org&#x2F;papers&#x2F;w21610</a>