Brain Death of an Idea: The Heritability of Intelligence

66 Pages·2014·0.462 MB·English

by  Manfred Velden

Manfred Velden
Brain Death of an Idea
The Heritability of Intelligence
With4 figures
V&R unipress
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Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
PartI:Themethodsfordeterminingtheheritabilityofmentaltraits,
inparticularofintelligence . . . . . . . . . . . . . . . . . . . . . . . . . 9
Quantitativegeneticstudies . . . . . . . . . . . . . . . . . . . . . . . . 9
Moleculargeneticstudies . . . . . . . . . . . . . . . . . . . . . . . . . 22
PartII:Researchontheheritabilityofintelligence–adarkchapterin
thehistoryofscience . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Galtonandtheregressioneffect–amomentousmisunderstanding . . 26
Eugenics–fromaplausibleideatoparanoia . . . . . . . . . . . . . . . 30
Immigrationandtheintelligenceofthenation. . . . . . . . . . . . . . 32
11+–Atragedyunnoticed . . . . . . . . . . . . . . . . . . . . . . . . . 34
Scientificracism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
TheEysenckcase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Moleculargeneticpeculiarities . . . . . . . . . . . . . . . . . . . . . . 51
Summaryandcomment. . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Introduction
Thequestionastothedegreetowhichintelligenceisheritableappearsplausible
at first sight and, after many decades of intense research about the topic, we
expect a clear answer, particularly because the pertinent science, behavioural
genetics,presentsitselfwithmany formulaeandfigures,quiteinthestyleofa
naturalscience.Butwhoever takesacloserlookatthepublisheddataandthe
scientific discussion about the topic will be disappointed.1 Not only do the
published values for the heritability of intelligence range from about 10%
(negligible) to 90% (higher than the ones for most bodily traits), but the
methodsusedtodeterminethemareutterlycontested.Quiteafewresearchers
generally question the soundness of applying methods developed in order to
predictbreedingsuccessinplantsandanimalstomentaltraits(e.g.intelligence)
inhumans.Itmustevenbeaskedwhetherheritabilityestimatesingeneralareof
anyscientificorpracticalworthatall.Attemptstodeterminetheheritabilityof
intelligence by means of molecular genetics, i.e. through an analysis of the
DNA2, a procedure by means of which the solution of the problem has been
announcedfordecades,haveyieldednovalidindicationswhatsoeverforage-
neticaspectofintelligence.3
In this situation, anybody interested in the issue cannot but form his own
opinion.Withoutawellfoundedopinion,heisatthemercyoftoatrulychaotic
situationcharacterizedbythemostdiverseandcontradictoryopinions,claims,
deliberations,and data.Awell founded opinion,however,requires a close ex-
amination of the fundamental premises and principles underlying the proce-
dures by which so-called “heritability coefficients” are being determined.
Without a firm grasp of these premises and principles, any idea about the
heritabilityofintelligence(andothermentaltraits)willremainobscure.Inmy
1 Popularscientificpublicationsshouldgenerallybeavoided.Theyhaveonlycontributedto
makingthediscussionmoreideologicalandscientificallyobscure.
2 DesoxyriboNucleicAcid,giantmoleculecontainingthegeneticinformation.
3 Themoleculargeneticproceduresusedsofararestillpoorlyunderstoodwhenitcomesto
interpretingresults.
8 Introduction
view,themainshortcominginthedebateabouttheheritabilityofintelligence,
andthecauseforthechaoticsituationcharacterizingit,mustbeseeninthefact
thatthosefundamentalpremisesandprincipleshaverarely,ifever,beenmade
explicit.
Inordertoavoidthatthereader,anticipatingwhatiscomingup,closesthe
book right away, let me stress that the procedures for determining the herit-
abilityofintelligencedonotrequiremuchmathematicalskill.Thefundamental
rulesofmathematicswilldo.Graspingthecruxofthematterwillnotprimarily
result from mathematically comprehending certain formulae (they are quite
simple) but from comprehending the contextual meaning of those formulae.
Thiscomprehensionmaynotalwaysbesimplebut,after30yearsofteachingthe
matter, I am quite confident that my explications will be understandable for
anyoneseriouslyinterested.Letitbestressedagainthatwithoutafirmgraspof
themethodologicalbasicsoftheheritabilityestimatesanydiscussionaboutthe
topicmustremaintotallysenseless.
Attheendofmyexplicationswewillseethatthereisanobviousdiscrepancy
betweenwhatcanbesaidaboutthetopicfromascientificpointofviewandwhat
isassumedbyabroadpublicandalsoamongmanyscientists.Inmyview this
discrepancymayonlybeunderstoodbylookingatthehistoryofthediscipline
dealing with the heritability of mental traits and mental functions, a history
goingfarbackintothe19thcentury.WhenIsaythatwearedealingwithoneof
thedarkestchaptersinthehistoryofsciencehere,bothastothescientificlevelof
the discussion and its social and political repercussions, the reader may an-
ticipatethatinthishistorythingshavenotalwaysbeenallthatscientific.Andas
wewillsee,thischapterhasinnowaybeenbroughttoaconclusion.Thesecond
andlengthierpartofthebookisthereforededicatedtoit.
PetraGlaubitzdeservesmythanksforpreparingthefigures.
PartI:Themethodsfordeterminingtheheritabilityofmental
traits, in particular of intelligence
There are two approaches by which a determination of the heritability of in-
telligencemaybeattempted:thequantitativegeneticone,wheretheheritability
is indirectly determined by comparing the resemblance among relatives of
different degrees of relatedness, and the molecular genetic one, where, as
mentioned, the determination is done by analyzing the DNA. The prevailing
notionsabouttheheritabilityofintelligence,asfarastheyrelatetoscientificdata
in the first place and not just to intuition, all come from quantitative genetic
studies.Themoleculargeneticproceduresarestillinakindoftestingphaseand
have, as mentioned, so far not yielded any reliable indications that there is a
geneticcontributioninvolvedinintelligence.Assomescientistsareconvinced
thatmoleculargeneticmethodswilleventuallystillrevealaconsiderablegenetic
roleintheformingofintelligence(asmallyetunspecifiableroleisassumedby
nearlyallscientists),thosemethodsaswellastheirpresentresultswillbetreated
here,too.Butfirstthequantitativegeneticmethodswillbeoutlined.Aswewill
see the “model” of quantitative genetics implies decisive fundamental as-
sumptions about the relation between genes and environment with regard to
their effect on a quantitative trait (intelligence in our case). Knowing these
assumptionsisindispensableforacoherentdiscussionofthetopic.
Quantitative genetic studies
Themethodsfordeterminingtheheritabilityofquantitative(continuous)traits
(quantitativegenetics)weredevelopedinagronomyinthe20sand30softhelast
century in order to render predictions of breeding success possible. Without
knowledge about the heritability of a trait (e.g. milk yield in cows) it is im-
possibletopredicthowmanygenerationsitwilltake(acrucialeconomicaspect)
beforeadesiredchangeinthattraitisattained.
Quantitativegeneticsdealswithdifferencesbetweenindividuals.Ifweallhad
thesamebodysize,i.e.,iftherewerenodifferencesinbodysizebetweenpeople,
10 Themethodsfordeterminingtheheritabilityofmentaltraits
therewouldbenopointinaskingabouttheheritabilityofbodysize.Theextent
to which individuals differ is given in quantitative genetics (and statistics in
general)bythesocalled“variance”.Inordertocomputethevarianceofatrait
measuredinanumber(n)ofindividuals,onefirstcomputesthemean(M),then
thedeviationofalltheindividualvalues(x symbolizesanyofthesesinglevalues)
i
fromthemean(x –M),thenthesquareofallthesedeviations((x –M)2),thenthe
i i
sum of all these squared deviations ((cid:151)(x – M)2), and finally the mean of the
i
squareddeviations,suchthatthevariance(V)isnowdefinedintheform
P
ðx ¢MÞ2
V ¼ i (1)
n¢1
Why the deviations from the mean are squared, and why n, the number of
measured values, is reduced by 1 need not interest us here. Alluding to the
described single steps in computing Vit is oftentermed “mean square of de-
viations”. It is obvious that with large differences between single values their
deviationsfromthemeanmustbelarge,too,asmustthereforebethevariance.
“Variation”isoftenusedasasynonymforvariance.“Standarddeviation”inits
exactstatisticalmeaningisthesquarerootofthevariance.
In quantitative genetics, the concept of variance is of decisive significance
becausethemodelofquantitativegeneticsisoneofapartitioning(analysis)of
variance,wherethevariancetobepartitionedisthe“phenotypicvariance”(V ).
p
Thephenotypicvariancesimplyisthevarianceofatrait(quantitativevariable)
presentandmeasurableinapopulationofindividualsforwhomtheheritability
ofthattraitistobedetermined.
Thisphenotypicvarianceiscausedbydifferentfactors(oftencalled“sources”
ofvariance).Inthesimplestofsituationstherearetwo causesthatcreatevar-
iance:(1)thefactthatindividualsdiffergenetically,and(2),thefactthattheyare
raised and live under different environmental conditions. Accordingly the
partitioningofthephenotypicvariance(V )canbewritten
p
V =V +V (2)
p g e
whereV isthatpartofthe phenotypicvariancethatiscaused bygeneticdif-
g
ferences,V theonecausedbydifferencesintheenvironment.4Intheliterature
e
4 The formula documents the fallacy of the assertion made by the great psychologist and
neurotheoreticianDonaldHebbthatitmakesnosensetoassignadifferentweighttogenes
andenvironmentbecausegenesandenvironmentrelatetoeachotherwithrespecttosome
behaviouraltraitasdolengthandwidthofarectanglewithrespecttoitsarea(Hebb,1980,p.
72),aviewoftencitedsince(e.g.Ehrlich,2000,p.6).Buttheeffectsofgenesandenvironment