The tree's trunk height tracks cognitive age — accumulated developmental progress — while the four glowing nodes mark the milestones of Piaget's stages: sensorimotor, preoperational, concrete-operational and formal-operational. Each simulated year, progress inside the current stage grows by a combined rate drawn from stimulation (enriching activities, novel input) and nurture (secure, responsive caregiving). Low nurture does more than slow growth — sustained neglect below a threshold lets a stage's progress decay, visualised as the branch above it dimming and shedding red particles, echoing how insecure attachment and chronic stress can undermine milestones already reached.
rate = (0.25 + 0.75·stimulation) × (0.25 + 0.75·nurture)
progress += rate · dt_years / stageDuration
if nurture < 0.25: progress -= (0.25 − nurture) · decay · dt_years
gap = chronologicalAge − cognitiveAge
- Stimulation level — variety and richness of activities and environment; drives the pace of new-skill acquisition, mirroring information-processing accounts of continuous cognitive growth.
- Caregiver nurture — responsiveness and emotional security, in the spirit of Bowlby and Ainsworth's attachment research; below roughly a quarter, progress can regress instead of just stalling.
- Time scale — how fast simulated years pass, so you can watch a full childhood-to-adolescence arc without waiting.
- Developmental gap — chronological age minus cognitive age: positive means development lags age-typical milestones (a delay), negative means it is running ahead.
Real-world relevance: developmental psychologists track exactly this kind of gap — between expected, age-typical milestones and a child's actual attainment — to flag when extra support, early intervention or a closer look at the caregiving environment is warranted.