Joseph M. Horn

Intellectual resemblance among adoptive and biological relatives: The texas Adoption Project

Intellectual and personality measures were available from unwed mothers who gave their children up for adoption at birth. The same or similar measures have been obtained from 300 sets of adoptive parents and all of their adopted and natural children in the Texas Adoption Project. The sample characteristics are discussed in detail, and the basic findings for IQ are presented. Initial analyses of the data on IQ suggest moderate heritabilities. Emphasis is placed on the preliminary nature of these findings.

Personality Resemblances in Adoptive Families When the Children are Late-Adolescent or Adult

Members of 220 families who had adopted one or more children from a Texas home for unwed mothers at least 14 years ago completed the California Psychological Inventory and the Thurstone Temperament Schedule. Consistent with other recent adoption studies in Minnesota and Texas, there was very little resemblance between parents and adopted children or between adoptive siblings (average correlations about .05). The presence of a biological relationship raised correlations a little, but only a little, to about .15, suggesting that much of the explanation for personality variation must lie in within-family environmental variation or nonadditive genetic effects. In an earlier study, young adopted children appeared to be better adjusted, on the average, than biological children in the same families. This was no longer true for the late-adolescents and young adults of the present study.

Aspects of the inheritance of intellectual abilities

Genetic and environmental influences on IQ were examined in various subgroups of the Texas Adoption Project sample, including subdivisions by age, sex, and socioeconomic status. Also investigated were the effects of sampling error, restriction of range, ureliability of measures,whether or not the adoptive parents had information about the biological mother, whether or not the biological mother had elevated MMPI scores, and differences between verbal and performance abilities. The principal method used was path analysis, and the heritabilities obtained fell mostly in the range of 0.30 to 0.65.

Personality resemblance in adoptive families: A 10-year follow-up.

Children from 181 of the 300 families of the Texas Adoption Project were recontacted after a 10year interval, at an average age of 17. They completed two standard personality tests, the Minnesota Multiphasic Personality Inventory (MMPI) and the Sixteen Personality Factor Questionnaire (16PF), plus a life events questionnaire and were rated by a parent on 24 bipolar trait scales. MMPI and 16PF scores were available from the earlier study for the adoptive parents, and MMPIs were available from the agency files for many of the adopted childrens birth mothers. Parent-child correlations and regressions, sibling correlations, and comparison of the means of adopted and biological children were interpreted as indicating a modest genetic influence on personality traits (narrowsense heritability, uncorrected, of about .25), a near-zero influence of shared family environmental factors, and a substantial contribution of idiosyncratic environment. The relative emotional and social adjustment of the biological and the adopted children had shifted since the time of the first study, to the detriment of the adopted children, but most still fell in the normal range. Studying adoptive families is one way of assessing the extent to which personality differences reflect genetic differences among individuals rather than differences in their family environments. Moreover, if parents who adopt children have one or more biological children of their own, one can compare the resemblance of genetically related and genetically unrelated pairs. If information about the personality of one or both of the birth parents of an adopted child is also available, one can see what the resemblance of parent and child might be in the absence of any explicity shared experiences or interactions between them. If this resemblance is appreciable for some traits, it provides direct evidence for the influence of the genes in accounting for variation on these traits. If some of this information is available on two or more occasions, developmental questions can also be addressed. For example, do the genes play an increasing or a decreasing role in accounting for individual variation in a trait as children grow older? The Texas Adoption Project (Horn, Loehlin, & Willerman, 1979) studied 300 Texas families who had adopted one or more children through a church-related home for unwed mothers. Parents and children in these adoptive families were given various IQ and personality tests. IQ and personality test scores were also available from the agency files for many of the adopted childrens birth mothers, from tests taken during their residence at the home some 3 to 14 years earlier. In this article we will report the results of a 10-year follow-up of many of the children in the

The Influence of Rearing Order on Personality Development Within Two Adoption Cohorts

There is an extensive literature on the relationship between birth order and psychological traits, but no previous study has investigated the influence of ordinal position on personality development within adoptive siblings. Such a design is important because it effectively separates the effects of biological birth order and rearing order. Here we report data from two adoption cohorts in which subjects were biological first-borns reared in various ordinal positions. Data were analyzed with reference to Sulloways (1996) evolutionarily based sibling rivalry theory of birth order effects. Between- and within-family analyses indicated that rearing orders influence on personality was very weak. The only clear difference was for conscientiousness, on which first-reared siblings scored higher. We draw possible implications for Sulloways theory and speculate upon an alternative, prenatal biological process that may produce birth order differences.

Personality resemblances between unwed mothers and their adopted-away offspring.

In a sample of 300 adoptive families there was a tendency for adopted children to be more extraverted and emotionally stable than biological children. For extraversion there was a low statistically significant resemblance between unwed mothers and their adopted-away children. Paradoxically, however, children of mothers with elevated Minnesota Multiphasic Personality Inventory (MMPI) scores tended to be rated as more emotionally stable than children of mothers with better adjustment on the MMPI. This latter finding was interpreted as suggested an interaction between emotional sensitivity and the early environment. According to this hypothesis, individuals with genotypes making them vulnerable to their environments could thrive in the warm climate of the adoptive families, but turn out relatively badly in the presumably less benign families in which the unwed mothers were reared.

The Texas Adoption Project: Adopted Children and Their Intellectual Resemblance to Biological and Adoptive Parents.

Intelligence test scores were obtained from parents and children in 300 adoptive families and compared with similar measures available for the biological mothers of the same adopted children. Results supported the hypothesis that genetic variability is an important influence in the development of individual differences for intelligence. The most salient finding was that adopted children resemble their biological mothers more than they resemble the adoptive parents who reared them from birth. A small subset of the oldest adopted children did not resemble their biological mothers. The suggestion that the influence of genes declines with age is treated with caution since other adoption studies report a trend in the opposite direction.

Opportunity for aggression as a reinforcer in mice

Male mice acquired, extinguished, and reversed a position response in a T-maze when the opportunity to attack a “victim” mouse was provided as reinforcer. Half of the Ss were also given a fight before each maze trial. These Ss, compared to Ss run without prefights, were superior in choice performance early during acquisition and reversal learning, and showed slower extinction. It was also found that preference for the aggression-rewarded side of the maze increased from first to second trials given on the same day.

Aggression as a component of relative fitness in four inbred strains of mice.

In four separate experiments, male mice from the RF/J, BALB/cJ. DBA/2J, and C57BL/6J strains were grouped in a seminaturalistic environment with females from the C57BL/6J strain. Observations were made concerning the aggressive behavior of the different strains, and starch gel electrophoresis of the esterase III and hemoglobin proteins was used to determine paternity of the offspring. Results indicated that the most aggressive strain, RF/J, sired 95.6% of all offspring. Control groups indicated that the reproductive disadvantage of the BALB and C57 males could be explained in terms of differences in general fertilizing ability, the effects of grouping mice together, or pregnancy blockage. These factors alone were not sufficient to explain the disadvantage of the DBA males. Differences in fighting success may therefore be an important determinant of fitness differences between RF and DBA males.

Antecedents of Children's Adult Outcomes in the Texas Adoption Project

Four composite variables concerning life outcomes were derived from a brief mail questionnaire describing 478 adults, now in their 30s and 40s, who had participated as children in the Texas Adoption Project. Responses were obtained from the participants themselves, their parents, and their siblings. MMPI scores of the parental generation were correlated with the adult outcomes of their biologically related and unrelated children. The obtained correlations were low, but for the biological relationships positive parent adjustment went with positive life outcomes of their children, whereas for adoptive relationships the reverse was the case. Favorable MMPI scores from late adolescence were favorably related to adult outcomes, as were favorable personality ratings from childhood.