Kulik (2004) concluded in his research that accelerated students did just as well as the bright students in the grades into which they moved. Also accelerated students have higher educational ambitions. When looking at the effect size we see that accelerated students surpass the performance of the non-accelerated students of an equivalent age and intelligence by nearly one grade level (d=0.88). One of the reasons it is one of the least used methods for gifted students is due to the typical claim that it is not beneficial from a social and interpersonal perspective. However, it seems that not accelerating students has a negative social impact.
NB: This effect size has dropped from d=0.88 in the Visible Learning book to d=0.68 in this unpublished synthesis. As with many areas of research, new findings can shift and change the balance of where an influence sits but the main findings on acceleration have not changed - however the magnitude of the effect has been moderated somewhat.