In a study comparing the memory spans of preschool and elementary school children, the latter group consistently scored better. This apparent increase in memory span with age could be explained partly by how

The older practise information more, they can process information faster

Explanation:

It is observed that as people get matured in age from their childhood that they are able to comprehend Information easier and faster, they process words easier to and can retain them well better than when they were younger. So from the question, at elementary school, the children become more efficient with details than when they were younger.

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.