Heredity and Environment in 300 Adoptive Families

The Texas Adoption Project

Joseph Horn

• Publisher: Routledge
• Year: 2017
• Print ISBN: 9780202363455
• eBook ISBN: 9781351515887
• Edition: 1
• Topic: Psychology
• Subtopic: Developmental Psychology
• Index: Psychology

This book presents the results of a thirty-five-year research project involving 300 families, each of whom adopted at least one child at birth from a Texas home for unwed mothers during the period of 1962-1970. The book weaves together information about the birth parents of the adopted children; information about the adoptive parents; and information about the children in these families.

Children adopted at birth have two sets of parents. Birth parents provide their adopted-away child with a genetic endowment, but do not participate in shaping the child's environment. Adoptive parents do not contribute genetically, but are otherwise in charge of directing the child's development. If adopted children grow up to resemble birth parents they have never seen, the clear inference is that hereditary factors have had an influence. Environmental factors are implicated whenever children resemble their adoptive parents, but not the birth parents. The Texas Adoption Project was designed to investigate the impact of genetic and environmental factors.

This unique and innovative longitudinal study is written for specialists and the educated public. An introductory guide is provided for the non-specialist reader explaining the form and statistical content of the tables. Additional technical material for specialists is contained in appendices. This important contribution to the literature on adoption will also be of interest to those interested in the relative weight of genetics and environment in human development.

1
A Brief History and Overview of the Project

This book is an account of a thirty-five-year research project involving three hundred families, each of whom adopted at least one child at birth from a Texas home for unwed mothers, the Methodist Mission Home of San Antonio. The book weaves together (1) information about the birth parents of the adopted children, gathered by the Home prior to the birth of the child in question; (2) information about the adoptive parents, gathered by our research team early in the study; and (3) information about the children in these families—the adopted child and others—gathered by the project at several points during their lives until adulthood.

1. Why Carry Out (or Read about) an Adoption Study?

Children adopted at birth have two sets of parents: both influential, but in different ways. The birth parents provide their adopted-away child with its genetic endowment but do not participate in shaping the child’s environment. The adoptive parents do not contribute at all genetically but are in charge of directing the child’s development through environmental means. If adopted children grow up to resemble the birth parents they have never even seen, the clear inference is that hereditary factors have had some influence. The Texas Adoption Project (TAP; Horn, et al., 1979) was designed to investigate this possibility across a range of human characteristics.

The question of the relative influence of the parents who provide the genes and the parents who provide the rearing environment is of direct concern to persons involved with adoptions: the adoptees themselves, members of families who adopt a child, and professionals who deal in one way or another with the adoption process. To what extent may an adopted child be expected to resemble one or the other set of parents in various respects?

But it is also a question of interest to anyone who wonders about the differences among individuals—the tremendous variation we all observe among the abilities, personalities, and life outcomes of the people we encounter. To what extent do these differences reflect differences among the genes of these individuals? To what extent do they reflect differences among the family environments in which they grew up? To what extent do they depend on other factors, such as unique combinations of genes and environments or sheer accident?

This book will not provide final answers to all such questions. No single study could. But it will provide many intriguing pieces of relevant evidence. We will not attempt to describe these in detail here: that is the task of the chapters ahead. But we can hint that the genes will play an appreciable role in our story, although not always a simplistic one; that there will be some surprises concerning how family environments operate; and that, at the end, ample mystery will still be left as to why humans grow up to be the fascinatingly varied individuals they are.

2. Previous Adoption Studies

There have been a number of previous studies that have used adoptive families to tease out genetic and environmental contributions to the differences among individuals. Readers who are not immediately concerned with this background may want to skip ahead to the next part of this chapter, “The Texas Adoption Project—Beginnings,” and return to review historical matters later if desired.

Most of the early adoption studies focused on IQ. The classic studies of Barbara Burks (1928) in California and Alice Leahy (1935) in Minnesota demonstrated a substantial contribution of the genes to individual differences in IQ; a study by Freeman, Holzinger, and Mitchell (1928) in Chicago emphasized environmental contributions. An Iowa study by Skodak and Skeels (1949) suggested both a substantial environmental effect on average IQ and a large genetic contribution to individual differences.

Studies that are more recent have also used the adoption design to address issues of heredity and environment with respect to IQ. These include a study by Schiff and colleagues (1978) in France, which compared small groups at two extremes: children born of low-status parents but adopted in infancy into high-status homes, and half-siblings of these children who remained with their low-status mothers. There was a substantial average difference between the groups, suggesting environmental effects on average IQ. Three Minnesota studies, by Fisch, et al. (1976) and Scarr and Weinberg (1976; 1978) obtained low IQ correlations between unrelated adoptive siblings and larger correlations between biological ones, suggesting substantial genetic effects on individual differences in intelligence. The recent and ongoing Colorado Adoption Project shares with our own study the advantage of having measured IQs for birth mothers, and goes beyond it in having measured IQs for a subgroup of the genetic fathers of the adoptees, plus extensive measurements of the adoptive family environment. The results of this study to date have been reported in numerous articles and in a series of four books (Plomin & DeFries, 1985, 1988; DeFries, et al., 1994; Petrill, et al., 2003). They suggest that genes have an increasing importance with age in accounting for individual differences in IQ.

The adoption studies have placed considerably less emphasis on personality than on IQ, but there has been some. A British study (see Eaves, et al., 1989, pp. 125ff) included measures of extraversion and neuroticism for adoptive relatives, as did a Minnesota study (Scarr, et al., 1981). In general, the results are suggestive of a moderate role for the genes, and a minimal role for shared family environment, but individual correlations are sometimes erratic; the sample sizes are small enough for this to be attributable to sampling fluctuation (see Loehlin, 1992, p. 32). The Colorado study measured several personality traits (Plomin, et al., 1998); at age 16 there was little evidence of either the genes or family environments contributing substantially to individual differences. The authors speculate that this may be due to personality having a substantial component of its genetic effects non-additive, that is, due to gene combinations rather than individual genes: the latter constitute the major factor accounting for biological family resemblance in typical adoption studies.

This argument receives support from a special kind of adoption study, the comparison of identical twins reared together and apart. The most famous example of this is the Minnesota study by Thomas Bouchard and his colleagues (e.g., Tellegen, et al., 1988), but there have also been studies in England (Shields, 1962), Finland (Langinvainio, et al., 1984) and Sweden (Pedersen, et al., 1988). In general, these studies agree with the ordinary adoption studies in finding very little evidence of the effect of shared family environment—the separated identical twins are nearly as similar as twins reared together; but the estimates of genetic effects are higher from separated identical twins than from adoption studies—identical twins are genetically identical and share gene configurations as well as individual genes.

The majority of studies of adopted children are not primarily concerned with individual differences, but with average levels of achievement or psychological problems in adoptees relative to children growing up in families in the usual way with their biological parents. There have been many such studies. Wierzbicki (1993) reviewed 66 studies of adoption outcomes, and van IJzendoorn, et al. (2005) reviewed 62 studies of adoptees focusing on IQ or school performance. Studies such as these are mostly concerned with whether adopted children do as well as children growing up in comparable biological families (not quite, although most fall in the normal range), or whether adoptees do better than comparable children left behind and reared by their biological mothers (on the whole, considerably better). Studies of this kind suggest that adoptive family environments are having an effect on the abilities, personalities, and adjustment of the adopted children. Why these environments apparently do not produce much in the way of lasting family resemblance is a topic to which we will return in later chapters of this book.

3. The Texas Adoption Project—Beginnings

The Search for a Cooperating Agency

While straightforward in conception, adoption studies often face two serious practical difficulties. First, results that inspire confidence require relatively large numbers of adopted children and adoptive parents who will cooperate with investigators over a considerable span of time. Second, agencies handling adoptions do not often make and record detailed assessments of the birth parents’ psychological status. Indeed, some of them see difficulties in placing children with adoptive families if such potentially negative information were to be generated. Could we locate an adoption agency who handled a large number of adoptions each year and who would be willing to share information and allow us to gather test data? In 1970 a search of all adoption agencies in Texas was begun. Using a list of approved agencies provided by the Texas Department of Human Services, a series of visits was initiated. At first results were not encouraging; visits in Dallas, Ft. Worth, and Houston showed that agencies were either not cooperative or had inadequate records. However, in San Antonio things were different.

The Methodist Mission Home (MMH) proved to be the ideal setting for the initiation of the TAP. Under the leadership of Dr. Spencer Stockwell, the MMH had been utilizing a wide range of objective psychological tests for guidance counseling over almost two decades. The number of unwed mothers served reached a peak of 321 in 1969 alone. Extensive files, including coded names and last known addresses, were being maintained on the thousands of girls and adoptive parents who had been clients over the years.

Of particular interest were the intelligence and personality tests given to the unwed mothers while they were in residence awaiting the birth of their child. These tests allowed us to take the measure of biological mother characteristics that are not usually available to investigators. It became obvious that if we could obtain the cooperation of the families who adopted these children we would have a good opportunity to correlate each biological mother to her biological but adopted-away child (an index of biological influences) and, at the same time, comparisons between adopted children and their adopted parents (an index of environmental effects) could be obtained using the same tests.

The way the MMH handled adoption also contributed to its suitability as a source for our sample of mothers and their adopted-away children. If the children have a long exposure to their birth mothers before adoption, both environmental and genetic influences from the mother are operating in the child’s development. This can reduce the power of an adoption study to do the very thing it was designed to do, namely, separate genetic and environmental influences. Fortunately, all MMH adoptive placements were completed within a few days following birth, thereby reducing the possibility that the adoption design has been compromised.

Another way the usefulness of adoption studies can be reduced is if the children are placed for adoption with relatives or with adoptive parents who have been selected because they are similar to the biological parents. Again, we were fortunate in that the few adoptions by relatives were identified in the records and could be excluded from the sample. Also, the only measure of biological mother—adoptive parent similarit...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Contents
• Preface
• 1. A Brief History and Overview of the Project
• 2. Intellectual Abilities in the Texas Adoption Project
• 3. Personality in the Texas Adoption Project
• 4. Achievement, Adjustment, Outcomes
• 5. Summary and Conclusions
• Appendices
• Glossary
• References
• Index