Innate

1. Our innate traits are shaped by both genetic and developmental variation

You can't bake the same cake twice.

Genetic differences contribute significantly to variations in psychological traits between individuals. However, an often-overlooked factor is developmental variation. Even with identical genetic instructions, the outcome of brain development can vary due to inherent randomness in cellular processes. This "noise" in the system can lead to substantial differences in neural circuitry and, consequently, in behavioral tendencies and capacities.

Sources of developmental variation:

• Molecular noise in cellular components
• Probabilistic nature of gene expression
• Contingent events in neural development

These factors mean that our traits are even more innate than heritability estimates alone would suggest. The genome encodes a range of potential outcomes, but only one unique individual is realized. This concept challenges the idea of genetic determinism while highlighting the intrinsic nature of our differences.

2. Twin and adoption studies reveal the heritability of psychological traits

MZ twins who have been reared apart are just about as similar to each other as ones who have been reared together.

Twin and adoption studies provide powerful evidence for the heritability of psychological traits. By comparing the similarity of traits between monozygotic (MZ) twins, dizygotic (DZ) twins, and adopted siblings, researchers can estimate the proportion of variance in a trait attributable to genetic differences.

Key findings from these studies:

• Most psychological traits show moderate to high heritability (40-80%)
• Shared family environment often has little to no effect on adult traits
• Non-shared environmental factors (including developmental variation) account for remaining variance

These results challenge the notion that our personalities and cognitive abilities are primarily shaped by upbringing or cultural influences. However, it's crucial to note that heritability estimates apply to populations, not individuals, and do not imply immutability or genetic determinism.

3. Genetic architecture of traits is complex and dynamic

There are no genes for complex psychological functions—there are neural systems for such functions and genes that build them.

The relationship between genes and traits is far more complex than a simple one-to-one mapping. Psychological traits are influenced by variations in many genes, often numbering in the hundreds or thousands. These genes typically do not have dedicated functions for specific traits but instead play roles in building and maintaining neural systems.

Characteristics of the genetic architecture of psychological traits:

• Polygenic: Influenced by many genes with small individual effects
• Pleiotropic: Individual genes affect multiple traits
• Dynamic: New mutations constantly enter the population
• Interactive: Effects of genetic variants depend on the presence of other variants

This complexity means that predicting an individual's traits from genetic information alone is challenging and imperfect. It also explains why these traits show continuous variation in populations rather than discrete categories.

4. Brain development is a self-organizing process influenced by noise

The genome does not encode a person. It encodes a program to make a human being.

Brain development is a remarkably complex process that unfolds through a series of self-organizing steps. The genome provides instructions for this process, but the outcome is not precisely specified. Instead, it emerges from countless cellular interactions and is subject to inherent randomness or "noise" at the molecular level.

Key aspects of brain development:

• Progressive differentiation of brain regions and cell types
• Activity-dependent refinement of neural circuits
• Influence of spontaneous neural activity before sensory input

This self-organizing nature means that even small initial differences can lead to significant variations in the final structure and function of the brain. It explains why identical twins, despite sharing the same genes and often similar environments, still show differences in brain structure and psychological traits.

5. Experience amplifies innate differences rather than evening them out

Rather than overriding or flattening out their effects, processes of brain plasticity may reinforce and even exaggerate the widespread initial differences that arise due to both genetic and developmental variation.

Brain plasticity, contrary to popular belief, often serves to amplify innate differences rather than minimize them. This occurs through several mechanisms:

• Self-selection of experiences: Individuals tend to seek out environments and activities that suit their innate predispositions
• Differential learning: People with different initial capacities may benefit differently from the same experiences
• Accumulation of effects over time: Small initial differences can lead to increasingly divergent trajectories

This perspective challenges the idea that nurture acts in opposition to nature. Instead, it suggests that our experiences are largely driven by our innate traits, leading to a positive feedback loop that reinforces initial differences. This process contributes to the increasing heritability of some traits, like intelligence, over the lifespan.

6. Personality traits reflect variation in decision-making parameters

There are no genes "for intelligence" or "for extraversion" or "for autism." There are just genes "for" building a brain.

Personality traits emerge from variations in underlying decision-making parameters, such as reward sensitivity, risk aversion, and threat detection. These parameters are influenced by the activity of neuromodulatory systems in the brain, which in turn are affected by genetic variation and developmental processes.

Components of personality:

• Basic decision-making parameters (e.g., reward sensitivity, risk aversion)
• Neuromodulatory systems (e.g., dopamine, serotonin pathways)
• Higher-order traits (e.g., extraversion, neuroticism)

This framework helps explain why personality traits show both stability over time and some flexibility in response to experiences. It also illustrates why genetic effects on personality are often indirect and why many genes implicated in personality disorders are involved in neural development rather than specific adult brain functions.

7. Intelligence is a general fitness indicator with a complex genetic basis

Intelligence reflects how well the brain is put together, how robust the genetic program of neural development was, and how efficient the resultant neural networks are.

Intelligence, as measured by IQ tests, appears to be a general indicator of the overall efficiency and robustness of brain function. This view is supported by several lines of evidence:

• Global correlates: Intelligence correlates better with global measures of brain structure and function than with specific regions or circuits
• Genetic architecture: Genes implicated in intelligence are involved in various aspects of neural development and function
• Correlation with other traits: Intelligence is associated with general health, longevity, and developmental stability

This perspective suggests that intelligence is not a modular trait but rather emerges from the overall quality of brain organization. It helps explain why intelligence is highly heritable yet also sensitive to environmental factors that affect general development, such as nutrition and education.

8. Sex differences in behavior have evolutionary and biological roots

If we think about human nature generally, then we should ask, first, whether it even exists.

Sex differences in behavior are observed across many species and have clear evolutionary origins related to different reproductive strategies. In humans, these differences manifest in various psychological traits and behaviors, including:

• Aggression and risk-taking (higher in males)
• Empathy and nurturing behaviors (higher in females)
• Interests and career preferences

These differences are rooted in biological processes, including:

• Hormonal influences during brain development
• Structural and functional differences in adult brains
• Direct effects of sex chromosomes on gene expression in the brain

While cultural factors certainly influence gender roles and expectations, the consistency of many sex differences across cultures and their presence in early infancy suggest a strong biological basis. However, it's crucial to remember that these are average differences and that there is significant overlap between the sexes in most traits.

9. Neuropsychiatric disorders are highly heritable and genetically complex

The idiopathic pools of autism (ASD), epilepsy (E), intellectual disability (ID), or schizophrenia (SZ) have been shrinking as more and more specific genetic causes are identified.

Neuropsychiatric disorders show high heritability, often in the range of 50-80%. Recent genetic studies have revealed that these conditions are caused by a complex interplay of genetic factors:

• Rare mutations with large effects (often de novo)
• Common variants with small individual effects
• Interactions between multiple genetic variants

Key insights from genetic studies:

• Many disorders share genetic risk factors
• Most implicated genes are involved in neural development
• The same genetic variants can lead to different clinical outcomes

These findings are changing our understanding of these disorders, revealing them to be highly heterogeneous conditions with overlapping genetic causes. This complexity explains why finding effective treatments has been challenging and suggests that personalized approaches based on genetic information may be necessary.

10. Genetic information raises ethical questions about reproductive choices

Genetic information doesn't have to be 100% accurate in predicting traits or disorders for it to be useful.

The increasing availability of genetic information is raising new ethical questions, particularly in the realm of reproductive choices. As we identify more genetic variants associated with traits and disorders, parents may have the option to select embryos or terminate pregnancies based on genetic predictions.

Ethical considerations:

• Rights of parents to make informed choices
• Potential for discrimination or eugenics
• Societal implications of widespread genetic selection

While genetic predictions will never be perfect due to the complexity of trait inheritance and developmental variation, they may still provide information that some consider actionable. This raises challenging questions about how to balance individual reproductive autonomy with broader societal concerns about equality and diversity.

Last updated: November 18, 2024