From one of his studies:
The position to be presented here grows directly out of Darwin’s (1871) view that natural selection endowed modern humans with larger brains, increased levels of general and social intelligence, and a more ethical and prosocial personality than ‘‘primeval man and his ape-like progenitors” (p. 159). Darwin wrote of increased levels of human qualities such as ‘‘courage, sympathy, and faithfulness,” and a ‘‘need for approval by others,” with a concomitant decrease in the frequency of ‘‘selfish and contentious people” who ‘‘will not cohere, and without coherence nothing can be effected” (p. 159). Darwin described how moral and inter-personal skills go hand in hand with the greater intelligence modern people possess.
Using structural equation modeling, he shows that personality traits are not orthogonal, and that a general factor can explain more than 50 percent of the variation in the lower-order traits.
To be really reductionist about it, people can be classifed into one of four categories: 1) smart-good; 2) smart-bad; 3) dumb-good; and 4) dumb-bad. And since intelligence and pro-social behavior are positively correlated, as Darwin suggested, there may be more people in categories 1 and 4 than in 2 or 3. This does seem consistent with a rough folk psychology. You could get even more reductionistic and say there are two categories: competent and incompetent, or something like that.
Rushton thinks of the general trait as "social efficiency." Another quote:
In a competitive world, there are always rewards (personal and professional) for more efficient persons—those who are more level-headed, agreeable, friendly, dependable, and open. We close by noting Tolstoy’s (1875/1918) famous opening in Anna Karenina, ‘‘All happy families resemble one another, but each unhappy family is unhappy in its own way.” Perhaps a similar principle applies to individuals: ‘‘All happy [or efficient] people resemble one another; each unhappy [inefficient] person is unhappy [inefficient] in his or her own way.”