Offspring Human Fertility Behavior in Biodemographic Perspective (2003) / Chapter Skim
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2. Genetic Influences on Fertility: Strengths and Limitations of Quantitative Inferences
2. Genetic Influences on Fertility: Strengths and Limitations of Quantitative Inferences
Pages 18-45
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From page 18... ...
VARIATIONS IN FERTILITY First, it is well established that there are strong psychopathological correlates of fertility. Thus, the number of children born to individuals with schizophrenia has tended to be below the general population's average (Tablensky, 2000~.
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From page 19... ...
Another change has been a major increase in the number of multiple births in most industrialized countries, this being a consequence of an increase in the use of various methods of assisted conception (Derom and Bryan, 2000~. Third, in many countries there are marked variations in average completed family size among ethnic groups.
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From page 20... ...
. RATIONALE OF QUANTITATIVE GENETIC RESEARCH STRATEGIES Geneticists have been aware for a long time that similarities between parents and children on some trait or even a familial loading on traits across a broader kinship do not allow an inference about genetic mediation.
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From page 21... ...
On the other hand, such features may be very informative because they could indicate the route by which genetic effects are mediated. Thus, it is not implausible that part of the genetic effects on population variance in fertility could derive from genetic effects on either physical attractiveness or age at puberty (although it is most unlikely that that is the only mode of genetic mediation)
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From page 22... ...
Evidence for violation of the EEA is available for life events in relation to the liability to depression and the effects of parental negativity, hostility, or criticism on antisocial behavior. That is, parent-child negativity toward each twin is more similar in MZ than DZ pairs, but even in MZ pairs the differences found in negativity are associated with within-pair differences in antisocial behavior.
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From page 23... ...
Because they apply to just a portion of the overall environmental effect, they could be influenced to a much greater extent than the overall proportion of the variance attributed to genes or environment. It is necessary, too, to go on to ask whether violation of the EEA is likely to apply to quantitative genetic studies of fertility or, more importantly, to the different traits that together make up the influences on fertility.
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From page 24... ...
With respect to genetic influences on population variance in fertility, there has been emphasis on the ways in which heritability has risen or fallen in an apparently meaningful way in relation to changed environmental circumstances (Kohier et al., 1999~. The findings are interesting, but there must be a good deal of skepticism about secular trends in view of the highly conflicting evidence from cohort studies that have examined changes over time in relation to traits for which there have been probably relevant changes in environmental circumstances.
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From page 25... ...
Gene-Gene Interplay Typically, traditional genetic analyses assume that all genetic effects are additive. However, it is known that in some circumstances there are synergistic interactions of a nonadditive kind.
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From page 26... ...
These should be tested for and, where relevant, included in the analyses. Second, as ordinarily presented, gene-environment correlations and interactions will be included in the estimate for genetic effects, despite the fact that they actually reflect the combination of genetic and environmental influences operating together.
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From page 27... ...
the effects of gene-environment correlations and gene-environment interactions simultaneously, it now seems that use of Markov chain Monte Cario techniques provides a way forward (Eaves and Erkanli, in press; Eaves et al., 2002~. Accordingly, it should now be possible to determine not only the overall genetic effect but also the extent to which this operates through gene-environment correlations and interactions.
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From page 28... ...
However, there are more limitations to adoptee designs than usually appreciated by behavior geneticists. To begin with, social agencies select prospective adoptive parents on the grounds that they will provide a good rearing environment.
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From page 29... ...
(2001) pointed out, Fisher's fundamental theorem of natural selection supposes that, for traits representing reproductive fitness, evolution will result in a progressive reduction of additive genetic variance so that, once evolutionary equilibrium has been reached, there will be no genetic effect on population variance.
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From page 30... ...
Unless further specified with respect to the various routes of genetic mediation, it means little and has no policy or practice implications. It may be accepted that there are significant genetic effects on a range of traits likely to be influential on fertility, including effects on social attachment (Brussoni et al., 2000)
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From page 31... ...
In considering how genetic influences might have effects, it is crucial to appreciate that multiple causal routes are likely to be involved and also that indirect chain effects are bound to be involved. For example, the relevant behaviors are likely to include the age of initiation of sexual activity, whether or not contraceptives are used effectively, the extent to which risk-taking behavior incorporates sexual practices that are likely to lead to wanted or unwanted pregnancies, the extent to which the establishment of careers is associated with deliberate postponement of childbearing, the intensity of sexual drive, the degree of affiliative behavior, the age at which cohabitation or marriage is first established, the number of cohabiting or marital partnerships (with the pressure that may be involved in having further children by a new relationship)
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From page 32... ...
It is also obvious that an enormous amount remains to be learned about the causal pathways that are most important. In that connection, probably the top priority should be epidemiological/longitudinal studies that span the age period from childhood to midadult life but with the additional requirement that such studies are used in an innovative fashion to test hypotheses about causal processes and, in particular, to use designs of sampling and data analysis that allow one hypothesis to be tested against competing hypotheses.
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From page 33... ...
Part of the limitation up to now has been the failure of most behavior genetics studies to include discriminating and sensitive measures of the environment. Third, research strategies need to be developed to test causal hypotheses about both secular trends and between-group differences in patterns of .
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From page 34... ...
Finally, molecular genetics research has a considerable potential both for providing invaluable leads on the possible nature of the neural processes underlying some behaviors and also for elucidation of patterns of naturenurture interplay (Plomin and Rutter, 1998; Rutter, 2002a, 2002b; Rutter and Plomin, 1997~. Although identification of specific individual susceptibility genes for particular behaviors can be expected to open up new avenues of potentially fruitful biological research, the identification of genes, in itself, will not delineate the responsible neural processes.
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From page 35... ...
Although almost anything is possible in biology, it does not seem plausible that this is responsible for the major ethnic variations in average completed family size or the fall in the same over time in most industrialized countries. ETHICAL ISSUES Some critics have raised concerns that the behavior genetics study of social behaviors and evolutionary theory will lead to a highly misleading impression of biological determinism that will bring about a neglect of research into social influences and, even worse, to a failure to act to alleviate or counter adverse social influences (Rose, 1995, 1998; Rose and Rose, 2000~.
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From page 36... ...
The first ethical concern, then, is that research could become dominated by a narrow mechanistic deterministic program. Of course, that is a possible danger, but the clear and unambiguous messages from genetic research lead in an entirely different direction.
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From page 37... ...
Possible conflicts of interest need to be made overt, not only at the point of initial ethical review and at the annual monitoring of research progress but also at the point of publication, as has been increasingly recognized and handled. With respect to genetic research, several main features warrant attention.
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From page 38... ...
Ethical review bodies provide an essential screening or advisory role, but, fundamentally, the ultimate responsibility that research be undertaken ethically, and reported ethically, must be placed on the individual scientist and, through that individual, to the person's employing authority. That is necessary because no set of rules, however exhaustive, can possibly anticipate all new ethical issues.
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From page 39... ...
Sometimes behavior genetics has been attacked on the grounds of being excessively reductionist in leading to a biological determinism (Rose, 1998~. The alternative is often portrayed as a need for a holistic approach.
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From page 40... ...
Behavior Genetics. Eaves, L., J
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From page 41... ...
Rodgers 1991 Secular trends in social class and sex differences in adult height. International Journal of Epidemiology 20:1001 -1009.
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Huckle 1990 Simulation of Mendelism revisited: The recessive gene for attending medical school. American Journal of Human Genetics 46:994-999.
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From page 43... ...
van den Oord 1999 Genetic and environmental influences on vocabulary IQ: Parental education level as moderator. Child Development 70:1151-1162.
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From page 44... ...
Caspi, and T.E. Moffitt 2002 Making Sense of Sex Differences in Psychopathology: Unifying Issues and Research Strategies.
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From page 45... ...
Greenwood In Twins as a natural experiment to study the causes of mild language delay: 2. press Family interaction risk factors.
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