Behavioral genetics research consistently shows that identical (monozygotic) twins exhibit the highest correlation in IQ scores compared to other familial relationships. Because identical twins share 100% of their genetic material, they provide the strongest evidence for the heritability of intelligence. While environmental factors also play a role, the genetic similarity of identical twins results in more consistent cognitive outcomes than those seen in fraternal twins or siblings.

In genetics, a recessive trait like blue eyes only manifests phenotypically if an individual inherits the recessive allele from both parents. Therefore, for a child to have blue eyes, they must have received one recessive gene from their mother and one from their father. This implies that both parents must carry at least one copy of the recessive gene, even if they do not express the trait themselves.

Modern psychology emphasizes that human development is not the result of a single factor. Instead, it is the complex, dynamic interaction between genetic predispositions (heredity) and the external influences and experiences encountered throughout life (environment).

Twin studies are the gold standard for estimating heritability because they allow researchers to compare the similarity of identical (monozygotic) twins, who share 100% of their genes, with fraternal (dizygotic) twins, who share approximately 50%. By observing differences in trait expression between these groups, researchers can statistically isolate the influence of genetic factors versus environmental factors in the development of complex human traits.

Judith Harris's 'The Nurture Assumption' famously challenged the traditional view that parents are the primary environmental influence on a child's personality. Harris argued that peer groups and genetic factors play a much more significant role than previously believed, suggesting that the home environment has surprisingly little long-term effect on the development of a child's personality compared to the influence of their social environment outside the home.

Environmental factors and training play a significant role in shaping a child's abilities, but they cannot drastically transform a child's fundamental cognitive potential. Intelligence is a complex trait influenced by genetics, environment, and various other factors. While enrichment can optimize existing potential, it does not fundamentally alter the baseline intellectual capacity determined by biological and developmental constraints.

Genes are segments of DNA that serve as the basic units of heredity. They carry the instructions for the development and functioning of living organisms, ensuring that traits are passed from one generation to the next.

Adverse environmental factors, such as chronic stress, malnutrition, or exposure to toxins, can have profound impacts on human development. These stressors can contribute to both psychological challenges (sometimes historically referred to as mental retardation in older literature) and various physical health issues. Therefore, both A and B represent potential negative outcomes of poor environmental conditions.

Developmental stages are often categorized by age ranges, though these can vary by theoretical framework. Early adulthood is generally considered to span from the mid-twenties to the mid-forties. During this period, individuals typically focus on establishing careers, forming long-term relationships, and achieving independence, marking a transition from the exploration of adolescence to the responsibilities of middle adulthood.

Developmental psychologists generally define middle adulthood as the period spanning from approximately age 40 to age 65. This stage is often characterized by career stability, family responsibilities, and the beginning of physical changes associated with aging, leading into the stage of late adulthood.