Until recently, comparisons of kids who were promoted and kids who were retained indicated that retention didn't seem to help academic achievement, and in fact likely hurt. So the best practice seemed to be to promote kids to the next grade, but to try to provide extra academic support for them to handle the work.
But new studies indicate that academic outcomes for kids who are retained may be better than was previously thought, although still not what we would hope.
A meta-analysis by Chiharu Allen and colleagues indicates that the apparent effect of retention on achievement varies depending on the particulars of the research.
Two factors were especially important. First, the extent to which researchers controlled for possible differences between retained and promoted students. Better studies ensured that groups were matched on many characteristics, whereas worse studies just used a generic "low achiever" control group. Second, some studies compared retained students to their age-matched cohort--who were now a year ahead in school. Other studies compared retained students to a grade-matched cohort or to the grade-matched norms of a standardized test.
Which comparison is more appropriate is, to some extent, a value judgment, but personally I can't see the logic in evaluating a kids' ability to do 4th grade work (relative to other 4th graders) when he's still in 3rd grade.
The authors reported three main findings:
1) studies with poor controls indicated negative academic outcomes for retained students. Studies with better controls indicated no effect, positive or negative, on retention versus promotion.
2) When compared to students in the same grade, retained children show a short term boost to academic achievement, but that advantage dissipates in the coming years. The authors speculate that students' academic self-efficacy increases in that first year, but they come to adopt beliefs that they are not academically capable.
This pattern--a one-year boost followed by loss--was replicated in a recently published study (Moser, West, & Hughes, in press).
The question of whether it's best to promote or retain low-achieving students is still open. But better research methodology is providing a clearer picture of the outcomes for these students. One hopes that better information will lead to better ideas for intervention.
Allen, C. S., Chen, Q., Willson, V. L., & Hughes, J. N. (2009). Quality of research design moderates effects of grade retention on achievement: A meta-analytic, multi-level analysis. Education Evaluation & Policy Analysis, 31, 480-499.
Moser, S. E., West, S. G. & Hughes, J. N. (in press). Trajectories of math and reading achievement in low-achieving children in elementary school: Effects of early and later retention in grade. Journal of Educational Psychology.