1 Psychology and human development1.11 Underlying concepts in human development
1.11.3 Methodology for studying development
2 min read1 figure
Written by Resident Doctor Committee · Published
Why design matters
Age cannot be randomly assigned, so comparing age groups only describes differences. Change, stability and sequence need repeated observations of the same people.
Cross-sectional and longitudinal designs
Design | Method | Strengths | Weaknesses |
|---|---|---|---|
Cross-sectional | Separate age groups measured once | Quick and cheap, with no years of follow-up or attrition | Age is confounded with birth cohort; shows no within-person change, sequence or cause |
Longitudinal | The same people measured repeatedly | Temporal order, within-person change, stability and sequence | Time, cost, selective attrition, practice effects |
The chief advantage of a cross-sectional design is speed and low cost, not causal, individual-trajectory or age-versus-cohort inference.
A longitudinal design is needed to test whether an early characteristic predicts a later outcome and whether a third variable moderates that link.
The British birth cohorts and the Dunedin study follow people over time to link early experience with later outcomes.
Natural experiments
A natural or quasi-experiment compares groups defined by an exposure that the investigators did not allocate and that was not randomised, such as a policy change, famine or adoption after deprivation.
It strengthens causal inference about an early exposure but cannot exclude residual confounding.
The English and Romanian Adoptees study compared outcomes after different lengths of institutional deprivation, but adoption timing was not random.
A timing pattern in outcomes from such a study can support a sensitive period.
Judging causation
Design | What it supports |
|---|---|
Cross-sectional correlation | Association without temporal order |
Prospective longitudinal | Temporal order, with possible confounding |
Natural experiment or genetically informative design | Stronger inference when assumptions hold |
Randomised intervention trial | Causal treatment effects |
Exposure before outcome, a dose-response pattern and replication strengthen an association, but none alone proves causation.
Adverse childhood experiences (ACEs)
The original ACE study, by the Centers for Disease Control and Prevention (CDC) and Kaiser Permanente, linked cumulative childhood adversity with later mental and physical ill health.
Risk rises in a graded, dose-response way with the number of adversity categories experienced, not with the type of adversity.
The same gradient holds for suicide attempt and for cumulative maltreatment.
Parental imprisonment is a recognised ACE category, and ACEs are linked to later delinquency and offending.
Retrospective reports risk recall bias; prospective evidence adds confidence, though individual outcomes vary.