"Memory span in humans has been intensely studied for more than a century. In spite of the critical role of memory span in our cognitive system, which intensifies the importance of fundamental determinants of its evolution, few studies have investigated it by taking an evolutionary approach. Overall, we know hardly anything about the evolution of memory components. In the current study, I reviewed the experimental studies of memory span in humans and non-human animals and the evolutionary hypotheses."

The magic number seems to be seven.

That's why, I suppose they have those numerical number tests in non pen and paper IQ tests.