Grasp a bunch of flowers in your hand, making sure you hold them towards the bottom of the bunch so that it splays out in a pleasing fashion, and you are well on your way to winning a lady’s heart, and to understanding Spearman’s law of diminishing returns.
The general factor of intelligence is strongest at lower levels of intelligence. It may be a case of “All neurones to the pump”. When abilities are low, most problems are difficult. In such cases, all resources have to be thrown at the problem. When abilities are higher there is more spare capacity for differentiation of abilities. Brighter persons have a lower proportion of their abilities accounted for by a common factor, even though the have higher absolute abilities.
So, if we stick to the flowers analogy in this post-Valentine’s day phase, the flowers of intellect of less able persons are tightly held together. The vector of “flowerness in common” runs from the bottom of the bunch of flowers to about two thirds up the bunch. In bright persons “flowerness in common” runs from the bottom to about one third up the bunch.
So, if you confine your studies of human beings to university students, not only will you misrepresent average mental abilities, but you will also diminish your measures of g, and be ever more likely to find apparently new, disparate mental abilities.
So, psychologists should study people who are not at university. I suppose that, more self-servingly, they might argue that everyone must go to university, simply to provide them with more representative study samples. However, the main effect of being at university is inebriation and delusions of adequacy, so it would probably be better to avoid university students altogether.
Flowers generally work, though.