Tolkien’s Middle Earth is a slow life history world. Hobbits mature at 33 and regularly live to 100. Numenoreans also mature slowly (Gilraen at 22 was at first considered too young to marry Arathorn) and live to between 100 and 200. At over 100, Dain was considerd a mere “stripling” at 32 when he killed Azog and the dwarves regularly live 250 to 300 years.
Furthermore, dwarves invest highly in their marriages: “For dwarves take only one wife or husband each in their lives, and are jealous, as in all matters of their rights”
All the things that are associated with slow life history should be associated with Middle Earth: the dwarves, hobbits, and the men of Gondor value their families and and Middle earth must be a highly stable environoment that changes slowly. All in all a highly "conservative" world, in the best sense of the word.
Most remarkable is the degree to which the dwarves match the ideas about genius described by Bruce Charlton in The Genius Famine. According to this book, the genius exhibits the endogenous personality:
“The Endogenous personality is the ‘inner’ Man; a person whose outlook on life is ‘inward.’ He is inner-directed, inner-driven, inner-motivated; one who uses inner modes of thinking, inner evaluations, in-tuition; one who is to a high degree autonomous, self-sufficient; one who is relatively indifferent to social pressures, influences and inducements”
In contrast to the exogenous personality:
“He stands in stark contrast to the Exogenous personality; that is, to most people. The Exogenous Personality is orientated toward the environment, particularly the social environment. These are people who want more than anything else social (including sexual) status, worldly success; people whose perceptions are directed outwards and who try to align their behaviour with group norms.”
Many dwarves exhibit this characteristic: “As for the men, very many also do not desire marriage, being engrossed in their crafts.” The endogenous personality is more common in men than in women and dwarves have an imbalance of men and women: “It was said by Gimli that there are few dwarf-women, probably no more than a third of the whole people.”
We can imagine that in dwarf society due to their high investment in marriage, marriage and children would be highly valued. Dwarf craftsmen would not have children, but would probably take apprentices at a young age, focusing not on reproducing but on passing down their knowledge. Furthermore, the ordinary dwarves, including dwarf women would be more endogenous, more focused on work and crafts than ordinary human beings. Dwarves would not be anti-social but dwarf socialization would be based around crafts and work.
The Genius Famine states “it was group selection which led to the evolution of geniuses.” In fact, the dwarves exhibit a high degree of group cohesion: after Thror was killed by Azog, all the dwarves, even those not descended from Durin began a massive war against the orcs: “Durin's folk gathered all their host, and they were joined by great forces sent from the Houses of other Fathers; for this dishonour to the heir of the Eldest of their race filled them with wrath.”
The dwarves are a species that relies on the products of genius rather than using genius occasionally, as is the case for humans: “There need not be many such men – indeed, there should not be too many, since the necessary mind is relatively unfit for the primary, day-to-day, activities of survival and reproduction of the species. But such men are needed – sooner or later, from time to time.”
It appears that about one third of the dwarf population is genius craftsmen, while the remaining two thirds raise a family in addition to their work. Actually, the number is fewer than this since of the dwarf women: “some desired none [husbands], some wanted one they could not have and would have no other.” This is the dwarven gambit: a species sustained by genius rather than by reproduction. In the harsh but stable environment of their mountain domains, the dwarves rely on the products of their craftsmen. Yet, there is a cost to produce such a large proportion of geniuses: “It is because of the fewness of women among them that the kind of the Dwarves increases slowly, and is in peril when they have no secure dwellings.”