We would rate karma as the most important variable in determining intelligence of individuals and this is because the other factors you cite are in essence karmic in nature as well. The genetic endowment is but one influence on things, but if you think about this more deeply, who you are with in terms of a family group and what that lineage has been involved in, their state of attainment with respect to becoming learned or remaining at a primitive level in understanding the workings of the universe, the more pragmatic needs for earning a living, and being a contributor to society on a high enough level to bring rewards of economic security, recognition, with much growth and satisfaction in progressing, are all interrelated.
You choose your family and they choose you. You will not jump from a past history of having a meager mental ability because you have been constrained in multiple lifetimes with such a condition, and then take a turn as a genius and becoming a professor at a top ivy league institution and a world-renowned scholar simply because you want to have something different in the current life. That opportunity, while technically always present, will not have the support of the past history through the genetic component as well as all the karmic influences stemming from how others have interacted with you and their energies continuing to do so even in a new lifetime. So there is always a kind of match with where one has been as at least an influence if not an overriding factor.
Similarly, the nature of one’s upbringing will be a function of who one is with, which has karmic roots in its determination as well as the behaviors the caregivers will manifest—how attentive and loving they might be or how stern and judgmental and harsh they might be in making demands that might be too punitive to be a positive influence and end up crushing the spirit of the young in the family. Prior lives in being a taskmaster may well translate into an individual becoming an overly stern father who might stifle ambition by suppressing initiative and destroying self-confidence of the offspring. The same is true of a woman who by nature is endowed with many traits as a caregiver and a nurturer, but if sidetracked because of life circumstances and too great a degree of impairment may have been part of a family that is dysfunctional and never given role models in effective parenting, and then has grown up in lifetime after lifetime as a similarly neglectful parent, may yet have that same pattern emerge in the current life. So if one is raised by such parents, their upbringing will have this underpinning of karmic influences to the negative. It could work the same way in a positive direction; if the karmic heritage of parents is one of lifetimes growing up in loving families, this will be part of the nature of the individual and their prior learning they will bring into a new incarnation inevitably, because each life is a continuation of previous lives.
By the same token, there are dark influences always pressing on human society at all levels and on the individual level explicitly, because this is where the deception and destruction begin. To undermine the young at the infant stage is a high priority of both the dark spirits who seek to gain entry into the energy of young children and babies so as to control them for the lifelong journey and gain in tapping their energy. And the extraterrestrials as well seek to manipulate and distort the thinking of the young to subjugate them in various ways, to envelop them in a state of complacency so they will simply be passive and not overreact to things not questioned and not strive to change the established order. That serves the Extraterrestrial Alliance quite nicely because they can maintain their grip on human institutions to render them ineffectual, working at cross-purposes, and promoting incompetence at all levels and misguided strategies and plans. This has held back human progress all through history and has been quite effective.
So any human being will receive a good measure of such negative influences quite directly within their beliefs at the deepest level of their subconscious, and this can hinder their intellectual development as well. This is also a goal in the manipulations of the Extraterrestrial Alliance—they have an ongoing program in fact to abduct every human child and poison the brain which both restricts the intuitive reach but also can influence the intellectual capabilities and other functioning of the mind, giving rise to various dilemmas such as learning disabilities and problems with attention deficit and hyperactivity. At extremes, this results in autism which is a partial or even total disconnection in the ability to communicate with the outside world and people are trapped in a body with a malfunctioning brain, and they may have a tremendous intellect and powerful mind in terms of their consciousness, but cannot express this very effectively in a body that keeps them more or less as a prisoner. So those are the dark extremes of what can go wrong.
There can be many positives because the current generation of human beings constitute a new version of human functionally, by virtue of the genetic endowment bestowed by Creator prior to the birth during the developmental stages. This new version of human is intellectually superior and gifted with a greater intuitive reach to have a much deeper understanding of things, even without being taught, because the intuition can reach out and find information on its own simply by having the desire to know. This is the greatest learning tool there is, but is only expressed to a meager extent in most individuals. The divine realm is working to correct this, but in turn has been countered by the Extraterrestrial Alliance with their campaign against the young through abduction and a dimming down of human young. So each individual is an end product of all the above influences to one degree or another directly, indirectly, small or large in their influence, respectively.
So here we are speaking of factors and influences only, not in any way the inherent capability and characteristics of that individual human being in terms of its soul and its potential. Each human being, no matter whether they are intellectually gifted or seemingly impaired, has a soul vastly greater and more capable than the level of function expressed as a human being. So the problems all stem from the human container and what life experience and the karmic influences, as well as the unpredictable overlay of corruption by the interlopers, create as a net end product in the state of being of each individual person.