This is such a fascinating subject that I’ve registered on this forum just to talk about it. I’m not an Objectivist, but I’m kind of friendly to Objectivism and admire Rand, and am usually the person who will defend her against the Yahoos who dis her in passing on online forums 
However, I do find there is something a bit odd in Rand’s reaction, and the reaction of some Objectivists, to some of the findings of cognitive science and evolutionary biology (and indeed evolutionary psychology).
If a basic premise is that we ought to have an accurate understanding of our own nature, then it seems to me that Objectivism should take these findings more seriously, and if they go counter to Objectivism, then Objectivism should be adjusted (and actually the adjustment is pretty minimal, imho). There seems to be a general feeling amongst Objectivists that if any of that stuff is accepted, some kind of floodgate is opened, and all the s**t will come pouring in. But evol biology/psych isn’t a form of psychological determinism.
What evol biology/psych tells us is that our bodies and brains are “designed” to cope with the kinds of situations that were typical during the long period (Upper Paleolithic, mainly) when we evolved to become the way we are today physically (i.e. our bodies and brains). That is to say, all the “intuitions” we have, the “unconscious” processes we have, are the precipitate of a whole bunch of working-out in the brain that we are not normally aware of. We have an intuitive sense, for example, of “folk physics” - our brains will steer our bodies in a way that takes for granted the existence of a physical world as that is normally conceived - more specifically, a physical world that includes about 20-30 other human beings, all of whom are known intimately, and who are extremely interdependent for their survival. This also means our “folk psychology” works a certain way - our intuitive sense of how people will react to what we do, for example. We also have an innate sense of “folk economics” - which is apt for small bands, with little in the way of property, no capital accumulation, etc. Of course, being evolutionary adaptations, there will be some bell-curve variation in the population with any of these traits - e.g. some will have a strong sense of “folk economics” that may override their rational understanding, e.g., they may feel the truth of a socialist form of economics so strongly that it overrides any rational economic arguments, but what they are feeling is simply the aptness of a socialist form of economic organisation in a small hunter-gatherer band with little property, no capital accumulation, etc., only they are superimposing that template on a large open society of relative strangers interacting, in which there are institutions like several property, capital accumulation, division of labour, etc. i.e. they are trying to “scale up” their intuition about what is just in economics, only they don’t realise that conditions are different from the kinds of conditions in which those intuitions may have had some validity, i.e. in the ancestral environment. This in itself explains the perenniality of “socialist” ideas and ideals (without having to demonize anybody - i.e. we can accept that such people mean well, and are in fact being true to their very own deepest intuitions, intuitions that may be as strong, for them, as the intuition to step out of the way of a looming object).
Put simply, our brains have a whole set of expectations about how the world will be. When the world isn’t that way, it causes us psychological discomfort. The kind of world our bodies and brains “expect” to exist is our “ancestral environment” - so, a hunter gatherer lifestyle, out in nature, with a few people known intimately. Even exercise can be looked at from this perspective: what our bodies expect is to walk a lot, and occasionally have bursts of extreme physical effort; also our bodies expect our diet to be inconsistent, so occasional fasting is in order.
It seems to me that this broad way of understanding ourselves is actually friendly to a properly rational way of looking at the world. If we are designed for a different environment, and (in evolutionary terms) we have only just ceased living in such an environment (the few thousand years since the Neolithic, say, are a drop in the ocean compared to the vast period during which we evolved to have the bodies and brains we have now), then the clash between many peoples’ intuitions and a properly rational understanding of the world is explainable, and even understandable (if not condonable).
Take “second-handedness”. If you think about the situation in the ancestral environment, it’s easy to see that we have a strong desire to “be famous”. If you are one of a group of 20 or so people, it’s easy to be “famous” in that group, to be recognised, to be seen and appreciated for what you are. In a large, open society of relative strangers, being “famous” in this way, being intimately known and recognised by others, is more difficult. This anonymity, this anomie, is a source of discomfort for many, and is I think the direct cause of “second hander” syndrome.
So what of the mind, you might say? Well, the mind is the most recent and glorious tool in the toolbox of humanity. If you think of the total bundle of human habits and traits as a sort of pyramidal or heirarchical structure, you have a whole bunch of regularities in the body that are designed to “cope” with the overwhelming constants in nature (gravity, temperature, air pressure, the necessity for fuel, etc., etc., etc.). These involve a lot of “hardwired” stuff, reflexes, etc. All these habits are rational in the sense that they are apt to the environment, they fit the ancestral environment (and the environment today, because as I said those are mostly physical constants). Then mid-way up the heirarchy we have the plasticity of our mammalian nature - mammals are somewhat adaptable, curious, questing. At this level are formed the sorts of habits that are evidently missing when animals are not reared with their conspecifics - so, mostly emotional signalling. Mammals’ main mark is that they are highly co-ordinated with their conspecifics. The emotional “centres” in the brain are highly developed - they signal their interior states to each other so that the group, set, herd, etc., can move as a whole in such a way that the individuals’ relative ability to reproduce is min-maxed. The environment these mid-level traits are “designed” to cope with are constants, not of nature, but of the species itself. Then finally, as a relative latecomer, riding on top of all these layered habits and tendencies, we have the mind. A development of the signalling process outlined above, but a development at a super-high level. IOW, mammals signal their interior physical states to their conspecifics (normal emotions, which tend to represent the body’s inner state to the outside world); what we as a species did, was develop this signalling process to a high level of refinement (language, symbol systems), at the same time interiorising external signalling (thought), and detaching the signalling process itself from its tie to bodily states (emotions). Here, also, is the beginning of conscious human society, born out of the possibility of lying that was always inherent in the signalling process. (i.e., if it’s possible for an animal to lie about its inner states, if it can detach what were previously automatic signalling processes - e.g. squealing when hurt, blushing, etc. - that might give it a reproductive advantage, but of course lie-detection, iow folk psychology, will also develop to counter that, in a kind of “arms race”, and that “arms race” progresses until we have something like the ability to talk to ourselves silently, and try out “possible ways the world is” in our minds, before committing ourselves to action based on any particular vision of the way the world is. IOW, we have thought, which proposes world models, possible-ways-the-world-could-be, and then selects them based on criteria of coherence with the set of established knowledge, plus correspondence with facts.)
So what we call conscious thought, i.e. talking to oneself internally, and working things out rationally, is a latecomer, but it’s an important latecomer that makes us highly adaptable, highly able to cope with changes nature throws at us, and much less dependent on hardwired traits. Kind of like a tugboat in relation to the vast ship that is the total bundle of bodily and emotional habits, traits, syndromes, etc. The rational mind is a relatively tiny subset of processes in the total economy of the total number of processes in the body and brain, but it is powerful enough to “nudge” behaviour in rationally apt (“fit”) directions in relation to changes in the world around us that are of higher frequency than the slowly-changing constants of nature, or the slightly more plastic, but still relatively stable constants of our species traits. This power actually comes at a high energy cost too (rational thought is energetically expensive, I mean in sheer physical terms) so in the past, in our ancestral environment, it was only pulled out of its holster at need (so to speak). And so we find it: people are actually happier when they don’t think too much. Thought is stressful and difficult - albeit hugely rewarding when we get it right, when the thinking is apt to the world (i.e. we figure something out, and are successful because of it).
In this broad picture, of course individualism as a philosophy is the latest, most advanced kind of philosophy. It’s like, in order to create the species mammal that we are, nature overbuilt us - our brains are these nonesuch things in the universe, that are in fact the most complex physical objects we know, superb steering mechanisms. (I should say, basically I am a pure physicalist, not a dualist, and I would call the “mind” what the brain does - i.e. the unique “style” of a body’s movements and activities through time and space. One of the things it does, of course, is internal dialogue, which we call “thinking” strictly so-called.) In building us, nature built better than she knew, so to speak, and so we find ourselves adventuring abroad in a universe we are superbly well-designed to cope with and master. Whereas in biological terms, genetic fitness is the min-maxed quantity, as individuals born indeed of that process, but not tied to it, we can be free as independent units, far freer than any other animal-as-individual.