A proper understanding of the biological origins of intelligence requires an understanding of what the term means

The Nature of Intelligence: Intelligence is the capacity for learning, reasoning, and understanding

  What Intelligence Tests Measure

  Intelligence quotient (IQ) scores are correlated with several desirable outcomes, such as income, education, and other traditional indicators of success.

 Critics say that intelligence is much broader than what tests measure. Such controversy over the meaning of intelligence makes finding biological origins more difficult

The Structure of Intelligence has been disputed.

 Lumpers believe that intelligence is a single, unitary capability

Splitters believe that intelligence is made up of several independent mental abilities

      The Origins of Intelligence

 The Brain and Intelligence

Brain Size does not determine intelligence across species.  Within a particular species, however, there is a moderate relationship. Notable exceptions: males’ brains are larger than females’ though they don’t differ in intelligence, and Einstein’s was smaller than the average female’s

 Men’s brains are larger than female brains, but this alone does not matter

 Neural Conduction Speed and Processing Speed

A correlation exists between IQ scores and reaction time

 IQ and nerve conduction velocity (NCV) are correlated even more strongly

 Processing Efficiency

Myelination of neurons, which is related to IQ, increases neuron conduction speed

Increased NCV may particularly affect the efficiency of working memory and short term memory, which is a better predictor of IQ than reaction time

Evidence of the role of neural efficiency is that higher IQ individuals use less brain energy

   Specific Abilities and the Brain

 Three capabilities have emerged as major components of intelligence:

Linguistic, logical-mathematical, and spatial

The linguistic area is mainly in the left frontal and temporal lobes

 Spatial ability depends on the interaction of somatosensory and visual functions with parietal structures, primarily in the right hemisphere

 Mathematical ability depends on two areas of the brain: The left frontal region and both parietal lobes

  Heredity and Environment both have clear influences on intelligence

Heritability of Intelligence

 Heritability increases from 20% to 60% over the life span

 IQ has a higher correlation among identical twins than fraternal twins, even when the identical are reared apart

         The Genetic Controversy

 Critics question the evidence for heritability of intelligence and fear the implication that intelligence is fixed. Research does not support either position

There has been a significant controversy over whether IQ differences between racial groups is genetically based; research also does not support this conclusion

        Environmental Effects

 The number and weakness of environmental influences makes it difficult to identify them

 Programs like Head Start have produced mixed results, but earlier and longer intervention is more promising

Though the IQ’s of adopted children are more closely correlated with their biological parents’ IQs than their adoptive parents’ adopting into a more stimulating environment can substantially increase IQ

            Disorders of Intelligence

Retardation is defined as an IQ of lower than 70 and difficulty meeting routine needs (e.g., self care). Many people are no longer classified in this way when they become adults

 There are four categories: Mild, moderate, severe, and profound

 Retardation can be caused by meningitis and prenatal exposure to alcohol or viruses, like rubella. The leading genetic causes are Down syndrome (caused by an extra ₂₁st chromosome), and fragile X syndrome

 Phenylketonuria is due to an inherited inability to metabolize phenylalanine

 Hydrocephalus is due to cerebral spinal fluid buildup in the cerebral ventricles

Autism occurs about one or two times in every 1,000 births, and typically includes compulsive, ritualistic behavior, impaired sociability, and retardation

 The Autistic Individual is retarded in about 80% of cases. They are impaired in communication, imagination, and socialization

 Much of the social problem of autistic people is believed to be due to a lack of theory of mind (the ability to infer what others are thinking)

   Autistic savants have some exceptional capabilities

  Brain Anomalies in Autism

 Damage is subtle and widespread, especially in the brain stem, the cerebellum, and temporal lobes

Prenatal exposure to the sedative thalidomide was associated with autism.

 Viral diseases are also believed to trigger an autoimmune response that interferes with brain development

  Biochemical Anomalies in autistic individuals include elevated serotonin levels and lowered levels of oxytocin.

  Heredity and Autism

The risk of autism in the identical twin of an autistic child is greater than 60%

Autism is two to four times more common in boys

Spectrum disorders suggest that autism is caused by several genes

     Effects of Aging on Intelligence

 Perceptual speed  declines after the age of 25, and numeric ability declines after 60

 Other abilities (e.g., reasoning, verbal ability) increase until middle age before dropping off