Additive Variance = The "Steady Nudge"
What it is: Tiny genetic effects that simply add up over time.
The Psychology: It dictates your baseline traits (like a natural tilt toward being introverted or smart).
How it develops: It actually gets stronger as you grow up. As you get more freedom, you naturally choose environments (hobbies, friends, jobs) that match your genetic tilt, making that original nudge even bigger.
Non-Additive Variance = The "Wild Card"
What it is: Unique combinations of genes that interact in unpredictable ways.
The Psychology: It explains sudden traits, extreme talents (like musical genius), or sudden mental health struggles that seem to come out of nowhere.
How it develops: It's the reason biological siblings can be total opposites. Because these complex genetic combinations get completely shuffled when a baby is made, you can't just predict the child's personality by averaging out the parents.
In short: Additive variance is why you slowly grow into your parents' traits. Non-Additive variance is why you are a completely unique individual.
The summary is built directly on foundation studies and major frameworks in behavioral genetics and developmental psychology.
The direct scientific sources for the key concepts used include:
1. For Additive Variance ($V_A$) & Increasing Influence with Age
The framework highlighting how additive genetic influence grows as a person creates their own environment stems from the concept of Active Gene-Environment Correlation ($rGE$).
Primary Reference: Plomin, R., DeFries, J. C., & Loehlin, J. C. (1977). Genotype-environment interaction and correlation in the analysis of human behavior. Psychological Bulletin, 84(2), 309–322.
Developmental Lifespan Focus: Plomin, R. (2014). Genotype–Environment Correlation in the Era of DNA. Behavior Genetics, 44, 439–444. (This foundational work documents how individuals use their autonomy to select, modify, and create environments correlated with their genetic propensities as they mature.)
2. For Non-Additive Variance & The "Wild Card" / Unique Traits
The idea that complex gene configurations create unique, non-linear psychological traits that do not easily "run in families" (explaining why siblings can be opposites or why genius/mental health issues spark suddenly) is known in psychology as Emergenesis.
Primary Reference: Lykken, D. T., McGue, M., Bouchard, T. J., & Tellegen, A. (1992). Emergenesis: Genetic traits that may not run in families. American Psychologist, 47(12), 1565–1577.
Focus: This landmark paper explains how non-additive genetic variance (epistasis and dominance) creates unique configural traits—like distinct personality quirks, creativity, or specific psychopathology—that are unique to the individual because the configuration is reshuffled during reproduction.