Is Early-Onset Major Depression Only Moderately Heritable at 40-50%?
This statement is partially true but underrepresents the full range—twin studies demonstrate that early-onset major depression has heritability ranging from 30-80%, making the 40-50% estimate fall within the moderate range but missing the substantial upper end of genetic contribution. 1
The Evidence on Heritability Range
The most comprehensive guideline evidence establishes that youth-onset depression shows heritability between 30-80%, with environmental factors explaining the remaining 20-70% of variance 1. This wide range reflects several important factors:
Methodological differences across twin studies, including varying definitions of depression, age ranges studied, and assessment methods used, account for the broad heritability estimates 1
Early-onset depression may actually show higher heritability than adult-onset forms, particularly in highly comorbid presentations, with some studies suggesting heritability up to 47% for early-onset cases compared to only 10% for late-onset depression 2
The 40-50% estimate cited in the question represents the middle-to-upper moderate range but fails to capture that many twin studies find heritability extending well beyond 50% 1
Why the Range Varies So Widely
Several critical factors influence heritability estimates:
Severity matters: More severe and psychotic forms of major depression show higher heritability (approximately 39%) compared to milder forms 2
Assessment method significantly impacts estimates: Depression diagnosed via structured clinical interviews captures unique genetic variance not captured by self-report measures, potentially explaining lower heritability estimates in studies using questionnaires 3
Measurement error inflates environmental effects: When accounting for reliability across multiple assessments, heritability estimates increase substantially—from approximately 40% with single assessment to approximately 70% when measurement error is properly modeled 4
Definition breadth affects results: Studies using broader definitions of major depression consistently show heritability of 33-45%, while those restricting to primary cases only show lower heritability of 21-24% 5
The Genetic Architecture
Twin, adoption, and family studies consistently demonstrate:
Genetic factors play a substantial but not overwhelming role in depression causation 5
Shared family environment contributes minimally or not at all to depression risk in most studies—the familial aggregation results primarily from shared genes, not shared household experiences 6, 5
Individual-specific environmental experiences (not shared by twin pairs) account for the non-genetic variance, including factors like poverty, negative family relationships, parental divorce, and child maltreatment 1
Clinical Implications
The characterization of early-onset major depression as "only moderately heritable at 40-50%" is:
Technically within range but misleadingly narrow, as it ignores substantial evidence showing heritability can reach 70-80% depending on methodology and phenotype definition 1
Most accurate for less severe, broadly-defined depression assessed via single self-report measures 5, 4
An underestimate for severe, early-onset, clinically-diagnosed cases, which show the highest genetic loading 2, 7
The statement is therefore "partially true"—40-50% falls within the documented range but represents a conservative estimate that doesn't reflect the full spectrum of genetic contribution, particularly for more severe early-onset cases.