Genetic and environmental basis of IQ

Galton - success breeds success in genetics

Used 'eminence' (someone who's famous/respected) as a measure of intelligence (only in males). 
- found higher chance of eminence the closer they were related to an eminent person (duh)
- found that adopted chidren never met the level of eminence of their adoptive family

So many things to roast this guy for, totally invalid and unreliable. 

Phenylketonuria (PKU) - single gene effect

Lack of tyrosine leads to PKU
- mental retardation often result

Tyrosine itself is coded by a single gene, thus the conclusion comes about that problems with a single gene can cause IQ impairments. 

Unlikely as limited variability (either retarded or intelligent)
- just because intelligence can be impaired by a single gene, doesn't mean intelligence is the result of a single gene

Alternative = Down's Syndrome (polygenic effect)
- chromosomal irregularity can effect multiple genes at once

Most reported genetic associations with general intelligence (G) are most likely false positives
(Chabris et al, 2012)

Tests of genetic similarity on IQ tests
- MZ twins share 100% DNA

Minnesota Study of identical twins reared apart (Bouchard et al, 1990)
- both had the exact same physical appearance (moustache, glasses, gait, occupation)
- scores on the WAIS correlated at .69 = very high
- identical twins reared together correlated at .88
- random people on ave. vary around 18 points whilst MZ about 6 points
Thus concluding that genetics play strong role in intelligence 

Could argue role of environment.

Accross studies, genetics accounts for 50% variance
- thus 50/50 split between genes and enviro?

General vs Specific IQ (80+years, MZ and same-sex DZ twins)
(Petrill et al, 1998)

• Results suggested genetic effects associated with G accounted for correlation between G and verbal, spatial and speed of processing abilities.
• No genetic influences found in specific cog abilities separate from G
• Contrastingly, memory more genetic from G than other cog abilities
• Comparisons with the young suggests genetic dedifferentiation in speed of processing

Texas Adoption Project (compared adoptive kids to birth and adoptive fam on WAIS)

• adoptive parents and own children, correlation of = .2(or a lil more)
• birth parents and own children = .3
• adoptive parents and adoptive children = .1
• adopted children and biological siblings = .3
• adopted children and adoptive siblings = 0
  However, this is soley based on the WAIS.

Heritability changes with age (McGue et al, 1993)

• shared envionment plays a role in childhood then declines sharply in influence
• genetic influence increases 

Genes and Enviro influence change with age (Bouchard and McGue, 2003)

• genetic variance for IQ increases with age
• enviro variance decreases 

Heritability Variation Due To Social Background (Turkheimer et al, 2003)

• Richer = less environmental influence on IQ
• Poorer = more environmental influence on IQ
  
• Age and SES are possible explanations

Overall = genetic similarity is strongly related to intellectual similarity.
(Grigorenko, 2000)

But...

• WAIS was the main measurement tool.
• Studied mostly white, middle-class, westerners.

Alternatively...

• Epigenetics = environment triggers off (or prevents the expression of) genetics
• Shown by how SES and age alter how much genes account for IQ (Turkheimer et al, 2003)

(1) How much is our intelligence determined by genes?

(2) Genes determine intelligence. Discuss.

(3) Environmental influences on intelligent are negligible. Discuss.