A new study in American Educational Research Journal ostensibly found that teaching advanced academic content in kindergarten is beneficial in terms of academic achievement as well as "social emotional skills": https://is.gd/FNLhjW. But a closer read fails to support those conclusions.
1. This is not a comparison of kids randomly assigned to academic vs. nonacademic conditions. In the kindergartens they looked at, virtually all teachers reported teaching academic skills that were coded as advanced (94% English/Language Arts, 99% math); the teachers' self-reports of how often they did some teaching of those skills were correlated with the outcome variables. "How often" was measured as number of days a month the teacher said she taught those skills, with no measure of how much time was spent at each session (which the researchers conceded) and also with no attention to how the teachers taught (e.g., via direct instruction vs. embedded in play or projects).
2. That drilling kids on content can boost test scores that same year (which happened here only to a moderate extent, incidentally) is unsurprising and means very little -- both because such tests are a lousy indicator of intellectual proficiency (in general, and particularly for young children) and also because a body of earlier early-childhood research shows that any such advantage tends to melt away after a few years.
3. The only potentially meaningful finding, then, concerns social-emotional outcomes. But...
a) The effects they primarily looked at dealt with compliance (self-control, persistence, attention, following rules), and there's reason to wonder whether these are always beneficial to the child, even if they're convenient for the teacher. The researchers did ask about internalizing (sadness, anxiety) and externalizing (anger, aggression) effects, too, but I'm not convinced these measures would have captured most of the potentially negative effects of developmentally inappropriate teaching, particularly if it took place for sustained periods and was done by direct instruction (which, again, we don't know).
b) All the social skills and behavioral effects were rated by the same teachers who taught the academic content, not by independent evaluators (let alone by independent evaluators blind to the type of instruction).
c) The ratings were all made that same year. Some long-term studies have found delayed negative effects of this kind of teaching.
d) Of 12 social/behavioral measures, there was a statistically significant (and quite small) effect on only three -- which didn't include internalizing or externalizing behaviors. So the most that could be claimed on the basis of this study is that, with respect to kids' aggression, anger, sadness, anxiety, etc., there was no short-term effect, positive or negative, as a result of teaching academic skills for an unspecified length of time using unspecified methods...according to the teachers themselves. And of course no attention was paid to the opportunity costs of academic instruction in terms of what the children didn't have a chance to do during the time they were being taught reading and math. Meaningful benefits of such instruction? Not shown here.