This is a big topic, and so this is a big post - the product of my thoughts over many years and many hours typing. I have considered writing a book about intelligence, and perhaps one day I shall. I am sure you will find this commentary as interesting as it is opinionated. It covers psychology, human development, education, and parenting - to add a speck of thought away from the theological and closer to the practical.
Nature or nurture is a topic has been debated much, with much research that allegedly 'prooves' this or that. I believe it goes beyond an academic level, to all areas of a person's life - mind, heart, personality, and character. I will not answer either one way or the other, but my answer will seek to illuminate the true hand-in-hand relationship between both external and internal influences on the intellect.
But a few caveats (qualifications) first: I am biased toward thinking it is our own decisions that make us intelligent or otherwise. Secondly, I am probably biased due to being a comparatively educated and intellectually successful person who has been raised by good parents. I suspect there are more than a few who would differ in opinion because they may not like the implications of what I'm about to say.
Those caveats being said, let us begin. Intelligence can be divided into roughly seven categories; linguistic, logical/mathematical, musical, visual, kinesthetic, interpersonal, and intrapersonal (see this page). I will define intelligence as the ability to learn - not the level of proficiency in each of the above categories - for I believe this should be the true measure of intelligence. Though I have no formal qualifications toward this topic, I believe my observations hold some truth. There are several crucial factors that I believe heavily influence intelligence. These factors are: intellectual stimuli, teaching, character, and metacognitive ability (explained below). I will discuss each of these below, but first, we must look at the concept of learning.
The essence of learning is shown in using two pieces of knowledge to determine another piece of knowledge. Leaning is like a brick wall, you use two blocks of knowledge you understand to create a block that rests on top. That block in turn can be used as the foundation for more. Simple. You cannot be told what the blocks of knowledge are, for memorisation of information is not nderstanding - each block can only be created by using only the blocks beneath. I believe this process of learning applies in every area of our seven or so forms of intelligence. Some people are better builders than others, and some intellectual walls soar with majestic elegance while others simply do the job, but why?
With few expections, most people are equiped a birth with a mind capable of the greatest feats of learning. At birth, our mind is remarkably adaptable - our thought processes are not rigidly formed and our brain must begin to make sense of the world. Even if a significant portion of our brain is lost or damaged at this age, the rest of the brain can relearn and fully compensate in most cases. And so our mind begins to form thinking paradigms which form what I like to learning strategies. These learning strategies are the processes by which our brain attempts to learn.
It is in the first few years of life that our brain develops these learning strategies. During these years the brain is effectively hardwiring core components into place, because the baby needs to use them reliably to develop functional understanding of themselves and their surroundings. I believe the brain also has a safety mechanism - these hard-wired core components are designed to be self-stabilising. Such stable systems are essential so that our brain doesn't get itself into trouble. The net effect of this self-stabilisation is that the more stable they become, the harder they are to modify.
So it is in the first years of life that none other than our parents (or our politically correct 'caregivers') provide us with the resources our brains require to form effective thinking systems. By the time we are 8, it is very difficult to change some essential thinking systems - and by the time we are 18, it is practially impossible. On this justification, the current educational system can be strongly critised. Millions of our dollars are poored at high-school level into trying to teach a world of imagery to the intellectually blind, and yet what is done for babies and toddlers when such vision would spark their mind to life?
It is the parents role to spark the mind of their child by providing stimuli to enable the intellectual growth of the child - this is the true role of a teacher. Intellegence can never be taught, for it is only learned. The role of a teacher is to help and guide and motivate the child on that path of learning. Only a part of that role involves being a fountain of knowledge which quenches the thirst of the child's questions, for most of a child's knowledge comes from other things. Children learn huge amounts when they play, when they talk, when they learn to kick a ball. Teachers must help them learn from the things they enjoy.
By the time a child reaches high-school, it is largely too late. At that age, the brain is largely hard-wired and self-stabilising toward the thinking systems that child has learned at a young age. And it is at this age where a third factor governs further development, character. Some may critisize this assessment, but I have seen ample evidence for it with my own eyes. It is those who have the determination, the patience, the self-disipline, and the vision who continue to learn. Essential elements of character are instilled in the child at an early age by non-other than the parents, and at this time its fruit is truely revealed.
We have all seen it too many times. The trouble student. The bully. The intelligent-but-ill-behaved brat. Sadly, it is largely too late for them also, for character can also become hard-wired in our brains. Without the motivation, drive, and self-disciple to learn, they see little purpose in their school work and seek other ways of filling their time. And there are all too many options these days, not only can one misbehave without fear of any real discipline - for gone are the days of the cane - but one can experiment with drugs and alcohol far more readily. Students know drugs and alcohol are not the answer, but they dull they question. In fact, once a student reaches the barrier of self-discipline, they tend to go one of two directions - to answer the questions, or to ignore the questions. Both work for a time.
Sometime between birth and death, there is a fourth critial element to learning intelligence that I believe is crucial. The earlier it is attained, the better, and I believe it too is learned and grown given appropriate stimulus from parents and teachers. This factor is the skill of metacognition. The term metacognition refers to the act of thinking about thinking, or the cognition of cognition. It is the ability for you to know what you are thinking, and thus control your own thoughts. Confused? Metacognition includes the ability for you to control, 1) person variables (knowledge about one's self, and others' thinking), 2) task variables (knowledge that different types of tasks demand different types of thinking), and; 3) strategy variables (knowledge about cognitive and metacognitive strategies for enhancing learning and performance).
I strongly believe metacognition is a learned skill, one which can be acquired if directed by good teaching - particularly during primary-school years. However, the class-room is no place for the individual guidance required to develop metacognition, and so it again falls to the parents. There is a strong correlation between metacognitive ability and academic success - and I suggest similar correlations with other categories of intelligence. It is only once a person understands what they are thinking that they can do much about thinking a better way, and seek to improve their learning strategies. Once a person becomes skillful at finding ways to improve their learning strategies, the process feeds back into itself, and the mind grows to be skillful not only at learning, but at learning to learn.
So as you can see, I believe internal (our nature) and external (nurture) influences work hand in hand in learning development. There is evidence to suggest our genetics influence our intelligence, and indeed in some congenital cases this is unquestionable. But I believe we should accept the things we cannot change, but change the things we cannot accept. Parenting, and its influence, has been given far too undervalued, and high-school level teaching programs have been grossly overemphasised as the answer to a deep-seated problem. The problem with our education is not in our schools, but in our homes - not in the classroom, but in the push-chair.