A child's physical maturity can play a critical role in determining whether he will benefit from retention. If the child is of small stature or underdeveloped, retention will allow an extra year for the child to grow and become similar in size to his classmates. Similarly, muscle coordination--how well the child can run and play with his classmates--may be a deciding factor. Age can be a negative indicator. If the child is already a little older than his classmates, holding him back another year will likely not make him a more successful student.
Emotional factors are the hardest to measure, but probably play a larger role than any other category in determining whether a child should be retained. If the child is immature socially, holding her back may allow her time to grow and mature and will allow her to better interact with her classmates in the future. If the child is struggling with an emotional issue of any kind, retention will likely not help, and the source of the problem should be the focus instead. Also, how the child feels about being retained, or whether or not she has been previously retained, may have a huge impact on her self-esteem and ability to see herself as a successful student.
Family factors can play a huge role in determining whether or not to retain a child. If the family is fairly transient, that is they move frequently for career or family reasons, retaining the child is probably not going to help. The child's issues are more likely associated with the disruption of moving and having to learn new schools and make new friends each time they move.
If the family is anchored, there are still other factors to consider about retention. If the child has brothers or sisters, and retaining him will put him in the same grade as or closer to another sibling, there could be long-term effects holding him back. If siblings are not an issue, parental involvement can be a determining factor. If the parents are involved in the school, they may help make the decision to retain a child, or at least have a rapport with the teacher that allows them to trust the teacher's decision.
Finally, don't exclude the child herself as a reason to consider retention. If the child has suffered a prolonged illness or been absent from school frequently, advancing her may actually cause her to struggle in higher grades. The child's exposure to the world around her--like sports, zoos, travel to other cities and states, or museums--can affect how well-prepared a child is to advance. Children who have had limited exposure to the outside world may struggle with comprehension, and retaining the child will not gain her any additional insight. Finally, the child's willingness to learn and do schoolwork should have an impact on the decision. If the child is not motivated to learn, retaining her will not make her any more motivated and may in fact squelch her initiative even more.