Development to Adult Mind

The human mind goes through significant, identifiable stages on its path to maturity. The age at which a stage occurs, the particular physical changes, and the personal experiences unique to that age affect the final shape of our adult mind.

Physical Stages

Both physical changes and behavioral insights indicate that we go through certain stages with critical periods on our road to mental maturity.

Brain Size

Figure 14.1 Human Brain Growth. More than doubles in 1st year and triples by 2nd year
Figure 14.1 Human Brain Growth

The brain more than doubles in weight the first year (Figure 14.1) and triples by the second year, then grows at a much slower rate, but with a regular pattern, until about age 20, remaining relatively constant in size until the 50s, after which there is a slow decline. This later age period is marked by changes not considered in the Mental Construction framework.

We are experiencing the world throughout the years of brain growth. Those lessons, formed with less cortical volume, continue forward and underlie our adult thoughts.

Critical Periods of Learning

At particular times in the path to our full mental capabilities, certain tasks must be mastered. If the opportunity is missed, the task cannot be mastered later, no matter one’s effort. A prime example is the ability to see. If an infant has an operation removing cataracts after the first two years of life, despite the optical information to the brain then being unimpeded, the critical period has passed and the child never learns to use sight as an input.

Language acquisition provides a critical period example in the mental realm. Children deprived of experiencing communication, like those few found locked into isolation rooms or feral children, do not develop language skills if it continues beyond age 10. If it ends earlier, they can develop rudimentary skills, but not facility.

Psychological skills, like friendliness, trust-worthiness, and self-assurance do not have such stark critical periods. These social skills are more subtle, harder to measure, and can be overridden by higher levels of cognition. However, these skills are being developed while the brain proceeds through a sequence of stages fixing the output from one area of the brain to another. Soft skills (psychological and social skills) once set become harder, though not impossible) to change when the myelination stage moves to another area of the brain.


In the physical arena, we note the routine pattern of myelination, which marks the completion of basic categorization of neurological connections in local modules of the brain. Myelination is the process of coating connections between more distant modules of the brain. This permits the learned categories to be shuttled easily and preferentially between modules. R. Douglas Fields in The Other Brain (p 282) describes the routine progress of myelin pathways as follows:

There is a curious pattern in the way myelination proceeds in the human brain after birth: the last regions of the brain to become fully myelinated are those involved in higher-level cognitive function. In the human brain, myelination proceeds in a slow wave from the back of the cerebral cortex (shirt collar) to the front (forehead) as we reach adulthood.

In a broad stroke, we develop categories in visual images first (the occipital lobe is in the rear of the brain above the shirt collar). Next, categories from hearing develop and are hooked together with visual categories. At that point, myelination continues across the top of the brain, where the bodily sensations arrive to the motor area, where bodily movement is controlled. Finally, myelination completes in mid-to-late 20s with delivery of fully categorized information about our environment, situation, and knowledge of our past to the prefrontal lobe, site of our executive functions.

Mental Stages

Stages in development are widely accepted and provide a typical approach in psychological and educational studies. Maslow’s Pyramid of Needs is broadest at what I have termed 3S Imperatives. Kohlberg’s Moral Development highlights that our morals shift over the years, with Erikson’s tasks of social development concomitantly occurring (Classic Stages of Individual Development).

Although development proceeds by stages, everyone does not achieve the same level of mastery, which becomes the basis for the next stage. Some people may be stymied as a child which distorts their mental development as a youngster and beyond.

Here, Mental Construction is concerned with the stages that precede the emergence of a mature adult mind.

The stages leading to mature adult thinking, listed in Figure 14.2, combine both biological changes and individual experiences of new types during the stages. The ages in this table are evocative rather than hard-and-fast rules. Each stage has specific physical changes, typical experiences and challenges which one meets with an ethical stance. Also each stage has internal resources that the prior stages have developed.

Figure 14.2 Biological stages occur with particular environmental events resulting in defining characteristics used by the next stage
Figure 14.2 Biological stages occur with particular environmental events resulting in defining characteristics used by the next stage


Ethics are basic. If it feels good, we want it to start or continue. If it feels bad, we want it to stop.

Newborns have strong muscles which they don’t know how to use. That is one of their first challenges, learning to control their muscles to do their bidding. In addition to growing, human infants must develop the understanding of the relation between their internal state and their body movements.

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Until we start learning language, everything we know is personal experience derived from our particular biological inheritance. It’s extremely rare for anyone to remember anything from those early years, yet from observation, it’s clear that we learn to satisfy our needs, such as satiety and comfort, through learned actions, such as squirming and crying. We discover that we can affect our environment through movement and noise.

Although we do not yet have the capacity to produce words, we understand that certain feelings followed by certain actions will result in certain outcomes. We detect and respond to patterns that we have learned to recognize. Although we lack words, we construct a very simplified worldview which we can manipulate.

Our ethics in this stage are basic. If it feels good, we want it to start or continue. If it feels bad, we want it to stop.


By the age of five, our hippocampus has developed sufficiently that long-term memory has come online.

The coating of its axons with myelin, a biological stage, between initial sensation and our reaction has been completed. This marks a significant stability in our assessment of external sense data to primitive internal worldview.

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Once we start talking with fluency and until we’re 5–7, our family life molds our daily existence. Our personal pain/pleasure measure is no longer adequate. We learn that satisfying parents is the route to happiness—good and bad become moral markers. Although our experience remains primarily a direct, with the addition of words, we learn the family’s organization of categories. We start developing second-hand knowledge, as occurrences which family members report become pieces in our worldview, although with less assurance than our own experience. However, with no experience in the areas they report, we rely on their experience.
If in infancy our needs were met, we are likely to give credence to experiences related by others. Our internal worldview of features grows and changes with new events, related experiences, and some basic relationships between events, actions, and results.
Distortions in internal worldview are possible and occur in multiple ways. If during childhood, parents are over-indulgent, praising their child’s actions whether merited or not, the child can develop a distorted view or reality that will wrap subsequent stages of mental construction. How? Later, constructive advice may be rejected because we have a deeper belief that what we do is ipso facto, right and the best.


Around the age of seven, we gain a new sense of self as the limbic system, our emotional system, completes its early maturation.

The biologically-controlled myelin sheaths connections between separate areas of the brain occurs. The capacity for situations and behaviors to satisfy or not our needs, goals, and fears are attached (learning) to occurrences, actions, and relationships with our internal worldview.

Click for moreSatisfaction of physical needs has been our driving behavior since birth, but now as a youngster it begins being tied to our knowledge of current circumstances and memory of past circumstances. With the semi-automatic linking of sensory environment and internal monitoring (through homeostasis), the amygdala categorizes the current situation with an emotional valence representing the satisfaction and likelihood of satisfaction of our many physical needs. If the likelihood of satisfaction is very low or very high, that emotional valence develops a strong connection to other limbic structures, which can trigger action before the information is passed upward to the frontal cortex for further processing.
In this stage, we are most affected by our family, neighbors, and peers at school and play—who are mostly like us—until we get broader social exposure in middle school, around the onset of puberty. Until then, we continue developing the categories we experience and those categories we hear about from those we know. We learn that we not only have a continuous existence, but that our existence is different from other people’s. We are unique. We may take pride in that, we may not, but we learn that we are distinctive from other people.
During this period, you probably learn that, although most people and households in your neighborhood are similar, they are not identical. You may discover that activities allowed at home, like drinking milk with your meal, are different at your friend’s house, where no milk is to be drank until the end of the meal. Similarly, your friends may be allowed to watch TV until 11 o’clock, while you have to be in your room with lights out at 9:30.
Your internal decision-maker develops a new ethical category. Your parents’ rules only apply at home. Only the rules of the house you are in must be followed. Rules have scope. They are neither universal nor absolute.
Some rules may have a relativity to them. Perhaps on one occasion a rule difference surprises you. You tuck that bit of knowledge aside. Going forward, you will know that there are rules you are unaware of. You might brace yourself to be unsurprised when hidden, unwritten, or unsuspected rules appear.
In grade school, you are learning new words and new categories for the immense possibilities of experiences. You are also introduced to new facts, which are outside of your direct knowledge, even beyond the second-hand nature of knowledge that your family and friends provide you with.
During these years, you are developing your internal social self (the categories you use in forming your personal worldview) which guides your choices among possibilities of action.


Sexual priority of choices.

The biological changes marking the start of this stage are commonly known as puberty. Although the changes are overtly physical, there is fundamental mental aspect too. The interest in sex becomes a primary filter through which we evaluate the world and situations around us.

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Also, leaving the familiar confines of neighborhood and grade school, we encounter a more diverse group of peers with different behavioral ranges. With the heightened interest in sexual matters and the desire for social acceptance, the urge to attract favorable attention becomes important.
We do not forget or ignore the categories and relationships we learned in the prior stages. How could we—they are built into the interconnections and synapses of our brains! Each stage starts with an understanding implicit in our brain’s neural arrangements. The lessons learned in the current stage, the neural patterns used to encode teenage experience, are the patterns we starte the stage with.
By this age, most of us have a fluency with language, so that we can express what we want—at least, when we know what that is. Consider this limitation on emotions and emotional needs. If a teen never heard words of love and warmth growing up, he/she will not have pathways that connect the abstract term “love” with emotional experiences. It will be very hard to think of love as it relates to him or herself, except in an idealistic sense, which is a severe handicap in a realistic pursuit of love and intimacy.
Language—the categories and relationships it assumes and expresses—continues to grow in importance in the teenager’s mind. In school, through books and the Internet, and through music, we experience indirectly the good, the bad, and the indifferent. We absorb or reject these messages.
Teens have time away from home rules, neighborhood norms, and strict oversight. Which rules do they follow? As they tend to adopt the “no house, no rules” strategy, they may try their hand at making their own rules. Some teens enamored of language search for logical consistency among rules. If there is a linguistic gap in a rule, they will exploit it.
Others with lower neural thresholds (more pliant Almost Gates) will see a profusion of different rules from those typically recognized. They may reject other people’s rule interpretations and insist on their own.
Parents, peers, teachers, and the media expose the teens to ideas, concepts, and approaches to future stages before the teens experience them. At this developmental stage, the teens learns second-hand expectations before they can independently evaluate them.

Young Adult

Physically mature, yet emotional choices are still developing

Superficially, by this life stage, one has completed their physical growth. If you are dissatisfied with your body, it’s time to exercise and adjust your diet, because the hormones and genetic factors have completed their basic outline of your physical appearance. Neurologically, senses and memory have completed their baseline preparation. The paths between the earlier modules and the initial decision sites of the prefrontal lobe are still being established (myelinated).

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From now on, as a young adult, you are able to decide which immediate behavioral choice will result in the greatest satisfaction of your goals and needs. If you are not under stress to make a decision immediately, you can pick and choose from among the many options available to you. If personal safety, satisfaction of physical needs, or sexual desire is of paramount concern, then you must pick the action that would immediately relieve that concern—long-term consequences be damned.
Although this has been stated in a deterministic fashion, “physical satisfaction” , “personal safety,” and “sexual desire” have different strengths in each of us. Those strengths arise from both genetics and experience. The acting strength is learned and relatively fixed during earlier stages. Whether you got milk as quickly as your stomach demanded, or your dirty diaper was removed at your first bellowing, the connections in your brain weren’t fixed yet. It was not until your limbic system solidified the connections between your physical state and the view of that reality formed as a youngster that your primitive mental world was set. During the teenage years, you started with the paths developed in the previous stage. By the end of the teenage years, the emotional pathways are reliably connected to the frontal lobe, which you learn to merge with your understanding of the immediate world.
As a young adult, you are able to weigh the immediate satisfaction of your emotional needs against the immediate consequences of those acts. As mentioned, when not under stress, you can even consider forgoing immediate concerns for long-term goals, which the current situation and your own preparation do not yet allow. You can consider actions that can bring about a future situation with your future capability that will allow you to satisfy goals that are beyond you now.
But that is an uncertain proposition. Your pathways to the most forward of the frontal lobes are still being formed and fixed during this stage. Some days, you will have more success with this than others; yet, whether you have success or failure, the pathways are being pruned by that outcome, with the biological clock awaiting the final flush of GABA-controlled learning and the cementing by myelin of the remaining axon paths.
You may still be at college or graduate school, or perhaps mastering your full-time occupation. In all cases, language continues to funnel your perceptions and verbal utterances into mental patterns and actual behaviors.
Culture and common mores have been seeping into your internal worldview since your first words. Now, with logic as well as knowledge, you lift your assessment of your culture to a rational level. You recognize societal norms and cultural ethics and apply reasoning to them. The purpose of rules becomes clearer to you, whether you care for those rules and their purposes or not. You consider how society and your goals align or don’t. You may agree with some, disagree with others, and find certain ones unimportant.
You also realize that you are on your own. Your decisions about rules have consequences. There is no absolute measurement stick that indicates whether you will abide by the society’s rules, ethics, mores, and taboos or not.


You have the power of firm beliefs and a loss of ability to change.

Myelination, the sheathing of axons cementing your idiosyncratic connections between your personal worldview and the executive areas of the frontal lobes, is finally complete. You think rapidly on a conscious level, deciding whether current situations are attractive for your goals and desires. If not, you consider other options available to you.

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You are able to more fully appreciate the trade-offs between immediate satisfaction of some needs and delayed gratification of others. In addition, you can understand the relation between your personal desires and societal dictates.
Every stage, including maturity, builds upon previous stages. You do not suddenly achieve wisdom.
Language and culture continue their role. Although they have already laid a groundwork of beliefs (categories and relationships), they continue to be a potent force for maintenance (or disintegration) of the facts that shape your internal worldview. In recent years the mass media stance on homosexuality, marijuana, cigarette smoking, unmarried mothers, and atheism has changed dramatically. Changes can be accepted or rejected, but they must be faced. Maturity does not mean that you must feel a certain way about changes. Maturity means that you will use your knowledge and values (3S Imperatives are forged by experience into values) to consider your reaction

Myelination marks the fixing of one’s preferred output from one brain module to another. In addition to speeding thoughts, it introduces rigidity into the internal worldview. It becomes more difficult to change or adjust to new situations. Thus, we typically resort to forcing new circumstances into categories based on previous situations rather than adding new categories.

Let’s consider the sources of thoughts and their net result—cognition.

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