Lifespan Development

This is Unit Eight of Psychology, Part One: designs, before birth, and thinking. Developmental psychology asks how people change across an entire lifespan, and three arguments run through everything it does. How inheritance and environment combine. Whether change is gradual or comes in distinct stages. And which characteristics persist while others get reshaped. Before any of the content, the unit does something the earlier units did not — it covers the study designs first. Because the two obvious ways of studying change over time each carry a specific problem, and one of them is a confound that looks exactly like development and is not. Then prenatal development in three stages, and the middle one matters most — because that is when the organ systems and the neural tube form, which makes it the period of greatest vulnerability. And the largest topic is a sequence of four cognitive stages, each identified by a specific capacity a child either has or does not — which is what makes the sequence testable rather than descriptive. By the end you will be able to do five things. One: name the three questions the field keeps asking. Two: say what a cohort effect is and which design it threatens. Three: name the prenatal stages and where vulnerability is highest. Four: name Piaget's four stages with their markers. Five: say what the zone of proximal development measures.

Four entries opening the unit. A definition, and then three questions that are not answered anywhere in this course — because they are the axes the whole field argues along rather than problems with solutions.

The field.

Psych 0851 Developmental Psychology

How people change physically, cognitively and socially across the entire lifespan. Entire is deliberate — development does not stop at adulthood.

The first question.

Psych 0853 Nature and Nurture

How inherited and environmental influences combine to produce a characteristic. Combine, not compete — the question is about interaction rather than about which one wins.

The second.

Psych 0855 Continuous and Discontinuous Development

Continuous development is gradual and incremental; discontinuous development proceeds through distinct qualitative stages. Different theories answer this differently for different capacities.

The third.

Psych 0857 Stability and Change

Which characteristics persist across the lifespan and which are reshaped by later experience. The answer differs by characteristic, which is why the question stays open.

Three entries on how you study change over time, and there are two obvious approaches with a problem each. One is fast and confounded. The other is slow and loses people. And the confound in the first is worth understanding because it produces findings that look like development.

The fast design.

Psych 0859 Cross-Sectional Design

Comparing different age groups at a single point in time. Quick, and every group differs in more than age.

The slow one.

Psych 0861 Longitudinal Design

Following the same individuals over an extended period, measuring them repeatedly. It removes the previous design's problem and takes as long as the question does.

And the confound.

Psych 0863 The Cohort Effect

A difference between age groups caused by the historical period each group lived through rather than by age. It appears as a developmental finding in a cross-sectional design and is not one.

Five entries. Three prenatal stages, and the useful thing is what each one is doing — because vulnerability follows the work. The stage building the organ systems is the stage where damage does most, and it is the shortest of the three.

The first stage.

Psych 0865 The Germinal Stage

From conception through implantation, roughly the first two weeks. Short, and mostly about establishing the pregnancy rather than building anything.

The second.

Psych 0867 The Embryonic Stage

From implantation through week eight, when the major organ systems and the neural tube form. Everything structural is built in these six weeks, which is why they carry the greatest vulnerability.

The third.

Psych 0869 The Fetal Stage

From week nine to birth, marked by growth and by the refinement of existing structures. Longest of the three, and refining rather than creating.

The threat.

Psych 0871 Teratogens

Any environmental agent causing damage during prenatal development. What damage is done depends heavily on which stage the exposure falls in.

One named case.

Psych 0873 Fetal Alcohol Spectrum Disorders

Lifelong physical, cognitive and behavioral effects caused by prenatal alcohol exposure. Named as a spectrum because the presentation varies widely.

Two entries on what arrives already working and what develops after. A newborn is not a blank — it comes with a set of automatic responses that support survival and then disappear as maturation makes them unnecessary.

What arrives working.

Psych 0875 Newborn Reflexes

Automatic responses present at birth supporting feeding and survival, fading with maturation. Their disappearance on schedule is itself a developmental marker.

What follows.

Psych 0877 Motor Milestones

A predictable sequence from head control to sitting, crawling, standing and walking. Predictable in order, and variable in timing — which are two different claims.

Seven entries, and this is the largest topic in the part. Four stages, and each is marked by a capacity a child either demonstrates or does not — which is what makes the sequence something you can test a child against rather than a description you apply.

The first stage.

Psych 0879 Piaget's Sensorimotor Stage

From birth to about age two, infants learn about the world through sensing and acting on it. Knowledge is what the body does, before it is anything else.

Its marker.

Psych 0881 Object Permanence

Understanding that objects continue to exist when they cannot be seen. It sounds trivial and it has to be acquired, which is exactly why it marks a boundary.

The second stage.

Psych 0883 Piaget's Preoperational Stage

Roughly ages two to seven, using symbols and language but not logical operations. Language arrives well before logic does.

A limitation of it.

Psych 0885 Egocentrism

Difficulty taking a perspective other than one's own. Not selfishness — a limit on what can be represented.

The next marker.

Psych 0887 Conservation

Understanding that quantity remains the same despite changes in shape or arrangement. Failing this test is the defining feature of the previous stage.

The third stage.

Psych 0889 Piaget's Concrete Operational Stage

Roughly ages seven to eleven, reasoning logically about concrete objects and events. Real logic, applied only to things that are actually there.

And the fourth.

Psych 0891 Piaget's Formal Operational Stage

From about age twelve, reasoning extending to abstract and hypothetical propositions. Which is what lets somebody reason from a premise they believe to be false.

Four entries. A different account of development, built on a measurement Piaget's stages do not make — what a learner can do with help, which is not the same as what they can do alone and is more informative about what comes next.

The measurement.

Psych 0893 The Zone of Proximal Development

The range of tasks a learner cannot do alone but can do with guidance. Testing solo performance alone misses this range entirely.

And it moves.

Psych 0894 The Zone Moves

The zone is defined by the gap between solo and supported performance, and it shifts as the learner develops. Which makes it a moving measurement rather than a fixed level.

The support.

Psych 0896 Scaffolding

Temporary support from a more skilled person, withdrawn as the learner becomes capable. Temporary and withdrawn are both in the definition — support that stays is not this.

And other people.

Psych 0898 Theory of Mind

Understanding that other people hold beliefs, desires and knowledge different from one's own. It is what egocentrism was the absence of, arriving as its own numbered capacity.

Five things you can now do, and Part Two takes up bonds and social stages. You can name the three questions the field keeps asking — how inheritance and environment combine, whether change is gradual or staged, and which characteristics persist. You can say what a cohort effect is — a difference caused by the historical period a group lived through — and that it threatens cross-sectional designs by looking exactly like development. You can name the three prenatal stages and say that vulnerability is highest in the second, when the organ systems and neural tube form. You can name Piaget's four stages with their markers: object permanence, then conservation, then logic about concrete things, then reasoning about the abstract and hypothetical. And you can say what the zone of proximal development measures — the gap between solo and supported performance — and that it moves as the learner develops.