Changes in Sibling configurations for American preschool children

Abstract

This paper uses data drawn from the 1940 through 1980 Public Use Microdata Samples of the U.S. Census of Population to document sibling configurations from the child's perspective. Changes in four aspects of siblings are examined for five cohorts of white and black preschool‐aged children: number, birth order distributions, spacing intervals, and sex composition. Changes in fertility behavior of adults in the post‐war era had a profound effect on the structure of sibling systems experienced by children. Successive cohorts of preschool children show a rise in number of siblings through the early post‐war years before showing sharp declines in number of siblings through the 1960's and 1970's. These shifts in size of sibling sets are reflected in changes in the proportion of each cohorts who are first born and only children, both of which have increased substantially by the 1980 cohort. The 1940 and 1980 cohorts have similar proportions of children with short intervals. However, the middle cohorts show the effects of the quickened pace of fertility with substantial proportions of children with comparatively short birth intervals. Finally, substantial shifts across cohorts in several measures of sex composition of children are observed. Most significantly, there is a marked decline in the proportion of children experiencing an opposite‐sex older sibling.     

Relative influences on recent changes in the first birth ratio in the United States

Researchers in psychology have focused a great deal of attention on the potential greater predisposition to achievement among first-born children relative to their siblings. Focusing on the United States as an example, a time series of the first birth ratio is used to show how the changing prevalence of first births relative to higher order births has altered the composition of birth cohorts, and the ratio is decomposed into four factors. Results show that the ratio increased significantly in the 1960s and early 1970s, but changed only slightly in the following decades. While more recent birth cohorts are composed of larger proportions of first-born children, the majority of children are still born as siblings. Contrary to expectations, the primary source of change was the proportion childless rather than decreasing higher order birth rates.     

The effect of birth interval on malnutrition in Bangladeshi infants and young children

This study was undertaken to investigate the independent effect of the length of birth interval on malnutrition in infants, and children aged 6-39 months. Data for this study were drawn from a post-flood survey conducted during October-December 1988 at Sirajganj of the Sirajgani district and at Gopalpur of the Tangail district in Bangladesh. The survey recorded the individual weights of 1887 children. Cross-tabulations and logistic regression procedures were applied to analyse the data. The proportion of children whose weight-for-age was below 70% (moderate-to-severely malnourished) and 60% (severely malnourished) of the NCHS median was tabulated against various durations of previous and subsequent birth intervals. The odds of being moderately or severely malnourished were computed for various birth intervals, controlling for: the number of older surviving siblings; maternal education and age; housing area (a proxy for wealth); age and sex of the index child; and the prevalence of diarrhoea in the previous 2 weeks for the index child. About one-third of infants and young children were moderately malnourished and 15% were severely malnourished. The proportion of children who were under 60% weight-for-age decreased with the increase in the length of the subsequent birth interval, maternal education and housing area. The proportion of malnourished children increased with the number of older surviving children. Children were at higher risk of malnutrition if they were female, their mothers were less educated, they had several siblings, and either previous or subsequent siblings were born within 24 months. This study indicates the potential importance of longer birth intervals in reducing malnutrition in children.     

Canadian fertility trends in perspective

This paper examines the broad movements of Canadian period and cohort fertility over the past 100 years, and compares them with corresponding trends in the US and other industrialized countries. The main movement in Canada was a decline in fertility extending from the 19th century to the present, interrupted in the 1940s and 1950s by a baby boom. Between 1871 and 1937 the total fertility rate (TFR) fell at about an average of 1.4% annually. The rate of fall in the US was similar, with the result that in the late 1930s the Canadian TFR was about 20% that of the US. The fertility boom that followed was steeper in the US than in Canada, and in the downswing that later followed, the rate of decline was similar in the 2 countries (3.4-4% annual average). But, the decline continued longer-- indeed still continues--in Canada, whereas the TFR in the US reached its lowest point in 1976. Moreover, the recent decline in fertility has been more severe in Canada than in almost any other industrialized country. The TFR relates to fertility in a single year and is highly sensitive to short-term changes in the timing of births. For the purposes of understanding and explaining long-term trends in fertility, the completed fertility rate (CFR) is a better index because it measures the ultimate family size of cohorts. Cohort fertility can be seen to fluctuate much less than does period fertility. In both the US and Canada, the peak cohorts, born in the early 1930s, had roughly the same completed fertility, and later cohorts continued to remain closely in step as the CFR fell sharply. In Canada the fall continues, though at a reduced rate, in the latest cohorts for which there is information. Apart from differences in amplitude, the dates of turning points and the shapes of the TFR and CFR curves of Canadian fertility are fairly similar. The long decline in cohort fertility is largely explained by the decrease in the proportions of families of 6 or more children. During the baby boom, for Catholics and non-Catholics alike, the proportion of ever-married women remaining childless fell by about 40%, the proportion having 2 children changed little, and the proportions having 3,4 and 5 children tended to increase substantially. The crucial difference between the 2 groups was in the proportions of women having 6 or more children. For non-Catholics, the proportion fell by over 4% from a high level. For non-Catholics, on the other hand, the proportion rose somewhat, though even after the rise, it was barely more than 1/2 the level to which the Catholic proportion had declined. Among Catholics, the effect of the massive decline in the proportion of women having 6 or more children was to swamp the effects of the increase in the proportions of women having 3, 4 and 5 children. The net effect was that fertility declined. Among non-Catholics, however, the comparable increases in the proportions of families of 3, 4 and 5 children, were not offset by any fall in the proportion of larger families, with the result that a baby boom occurred.     

Parental preference for sons and daughters in a Western industrial setting: evidence and implications

This paper considers whether sex composition of existing children in Australian families is an important factor in parity progression. Using census data from 1981, 1986, 1991, 1996 and 2001, women are linked with their co-resident children, allowing investigation of family sex composition and its changing impact over time on the propensity to have another child. The study finds that parents are much more likely to have a third and fourth birth if existing children are all of the same sex, indicating a strong preference for children of both sexes. This increased propensity has added around three per cent to the fertility of recent cohorts. The paper concludes with a discussion of the potential impact of sex-selection technologies on fertility. The authors argue that future widespread use of reliable sex-selection technologies might act to increase fertility in the short term, but would lead to a long-term reduction in fertility.     

Influence of variation in birth weight within normal range and within sibships on IQ at age 7 years: cohort study

ObjectiveTo examine the relation between birth weight and measured intelligence at age 7 years in children within the normal range of birth weight and in siblings.DesignCohort study of siblings of the same sex.Setting12 cities in the United States.Subjects3484 children of 1683 mothers in a birth cohort study during the years 1959 through 1966. The sample was restricted to children born at ⩾37 weeks gestation and with birth weights of 1500-3999 g.ResultsMean IQ increased monotonically with birth weight in both sexes across the range of birth weight in a linear regression analysis of one randomly selected sibling per family (n= 1683) with adjustment for maternal age, race, education, socioeconomic status, and birth order. Within same sex sibling pairs, differences in birth weight were directly associated with differences in IQ in boys (812 pairs, predicted IQ difference per 100 g change in birth weight =0.50, 95% confidence interval 0.28 to 0.71) but not girls (871 pairs, 0.10, −0.09 to 0.30). The effect in boys remained after differences in birth order, maternal smoking, and head circumference were adjusted for and in an analysis restricted to children with birth weight ⩾ 2500 g.ConclusionThe increase in childhood IQ with birth weight continues well into the normal birth weight range. For boys this relation holds within same sex sibships and therefore cannot be explained by confounding from family social environment.

What is already known on this topic

IQ at school age is linked to birth weight among low birthweight babiesSome evidence suggests the association might also apply to children of normal birth weight

What this study adds

IQ at age 7 years is linearly related to birth weight among children of normal birth weightThe relation was not due to confounding by maternal or socioeconomic factorsIQ is also associated with differences in birth weight between boy sibling pairs but not girls     

Sex ratio in Down syndrome

Data from 55 publications providing the sex ratio (SR), i.e. ratio between male and female cases of Down syndrome (DS), are presented. In general, SR was skewed toward an excess of males in the majority of studied populations, either in populations with a high level of cases ascertainment (epidemiological studies) or in selected groups. No significant correlation involving the age of either patients or mothers was found. Some other factors which might influence the sex ratio in DS at birth are mentioned. Meta-analysis of data from epidemiological studies suggests the phenomenon is not restricted to free trisomy 21 alone but appears in translocation cases, both in mutant and inherited translocation carriers (SR = 1.31 and 1.36, respectively). In contrast to nonmosaic 47, +21 cases, where SR is close to 1.3, an excess of females was observed in mosaics 46/47, +21 (SR = 0.83). No male predominance was found among patients with DS not tested cytogenetically (SR = 0.98), which may be explained by female predominance in false-positive cases. In populations with a fraction of clinically diagnosed cases of 30% and over, SR has intermediate value of 1.1. The ratio showed a tendency to increase since 1940's, reaching a mean value of 1.35 in 1980's varying from 1.3 to 1.62 in different populations), which might be a consequence of the growing use of karyotyping to confirm diagnosis and of a real increase in proportion of males. In the 1990's, the ratio fell to 1.22 varying from 1.03 to 1.27. As SR is assumed to reflect a proportion of paternal contribution, the discrepancy between the proportions of paternal errors in cytogenetic studies on parental origin of the extra chromosome (24% in the 1980's) and in molecular studies (5-10% in the 1990's) discussed in the literature might be explained by temporal changes alone. Genetic mechanisms of male predominance in trisomy 21 are reviewed, among them models for joint segregation of chromosome 21 and Y chromosome in spermatogenesis, and the chromosome 21 nondisjunction during 2nd meiotic division of oogenesis caused by Y chromosome-bearing spermatozoa.     

Social,biological, and political considerations on fertility in Arab populations

In analyzing fertility in the Arab countries, crude birth rate, total fecundity rate, and age specific fertility rates were measured. The data was obtained from United Nations, UNICEF, and the World Bank. In the early 1980's 13 of the countries had birth rates 40/1000. The majority of countries showed a decline in their crude birth rate (CBR) between 1960-83, except Somalia, which increased. The United Arab Emirates (UAE), Tunisia, Lebanon, and Kuwait, had the largest CBR decreases, followed by Morocco, Egypt, and Saudi Arabia. The global fecundity rate (GFR) shows the number of expected births a woman lives through her reproductive period, having children at the prevailing rate for each age. The GFR in these countries is much higher than those of non Moslem countries in the area. Results show that the fertility of Arab countries are in a gradual decline, but remain high, and many have a CBR over 40/1000. In the last 20 years Saudi Arabia, with the largest population of oil producing countries, has had a decreasing CBR. It is not in agreement with its high GFR, but this can be attributed to the large number of immigration workers in the country. The UAE showed a decrease in CBR from 46/1000 to 27/1000, the largest decrease in these countries. This decline coincided with the economic development due to oil production. Kuwait had a 25.5% decrease in CBR but less than Tunisia and Lebanon. The fertility decline in Kuwait intensified in the middle 1970's; the decline in northern Africa began in the late 1960's. There were declines in birth rates in the North African countries in the early 1970's except for Tunisia. The rapid declines in fertility can be attributed to the countries' socioeconomic and political situations.     

An examination of factors influencing black fertility decline in the Mississippi Delta, 1880-1930

Although the fertility decline in the black population in the Mississippi Delta between the late 1870's and early 1930's closely paralleled that of the national black population, it rose much more dramatically in the 1940's and 1950's to almost 1880 levels. Given the especially rural and oppressed conditions of blacks there, the initial decline seems puzzling. Low fertility rates in the 1930's reflected a large proportion of childless females. Investigations of changing contraceptive usage and mate exposure suggest both were minor components at most. Several physiological impairments were investigated including dietary deficiences, malaria, tuberculosis, and sexually transmitted diseases (STD). Evidence suggests STD played the major role, facilitated by nutritional and other health problems. Models relying heavily on those developed by McFalls and McFalls (1984) suggest 50-80 percent of the decline could have been due to the spread of STD. Age-specific birth rates for different periods and post-World-War-II fertility increases seem consistent with this finding.     

Population increases in obesity appear to be partly due to genetics

Studies have documented substantial increases in obesity throughout most of the industrialized world in recent decades. The majority of explanations for these increases have centred around environmental factors such as the increasing availability of high-fat, high-carbohydrate foods and sedentary lifestyles. This study sought to determine if genetic factors might be contributing to the increases in the proportions of North Americans who are obese and overweight. The body mass index (BMI) for a large sample of two generations of United States and Canadian subjects was correlated with family fertility indicators. Small but highly significant positive correlations were found between the BMIs of family members and their reproduction rates, especially in the case of women. For instance, mothers in the sample (most of whom were born in the 1940s and 50s) who were in the normal or below normal range had an average of 4.3 siblings and 3.2 children, compared with 4.8 siblings and 3.5 children for mothers who were overweight or obese. When combined with evidence from twin and adoption studies indicating that genes make substantial contributions to obesity, this study suggests that recent increases in obesity are partially the result of overweight and obese women having more children than is true for average and underweight women. It is speculated that improvements in medical treatments for conditions associated with obesity--particularly diabetes and heart disease--are making it possible for overweight women to live longer and to be more fertile than was true historically.     

Differential reproductive success and facultative adjustment of sex ratios among captive female bonnet macaques (Macaca radiata)

In a group of captive bonnet macaques (Macaca radiata) housed at the California Primate Research Center, variance in reproductive success among females is primarily due to differences in infant survival. The infants of low-ranking females have a smaller probability of surviving to 6 months of age than do the infants of other females. In addition, the juvenile daughters of low-ranking females are more vulnerable to behaviourally induced mortality than are other immature animals. Observational evidence indicates that this mortality is the direct result of aggression by unrelated, higherranking adult females. Although infants' sex is not consistently related to survival, yearly fluctuations in the survival of male and female infants are reflected in the extent and direction of the skew in the sex ratio of offspring produced the following year. Years in which the highest proportion of male infants survive are followed by years in which the largest proportions of the birth cohorts are composed of males, and years in which the largest proportions of females survive are followed by years in which the largest proportions of birth cohorts are composed of females. For infant females the probability of surviving is reduced when a substantial proportion of the birth cohort is composed of females. The same pattern is evident among the sons of low-ranking females. The adaptive significance of behaviourally induced variation in reproductive success among females is considered in relation to these data.     

Can Sibling Sex Ratios Be Used as a Valid Test for the Prenatal Androgen Hypothesis of Autism Spectrum Disorders?

Background

Sibling sex ratios have been applied as an indirect test of a hypothesized association between prenatal testosterone levels and risk for autism, a developmental disorder disproportionately affecting males. Differences in sibling sex ratios between those with and without autism would provide evidence of a shared risk factor for autism and offspring sex. Conclusions related to prenatal testosterone, however, require additional assumptions. Here, we used directed acyclic graphs (DAGs) to clarify the elements required for a valid test of the hypothesis that sibling sex ratios differ between children with and without autism. We then conducted such a test using a large, population-based sample of children.

Methods

Over 1.1 million subjects, born in California from 1992–2007, and identified through birth records, were included. The association between autism diagnosis, determined using the administrative database of the California Department of Developmental Services, and the sex of the subsequent sibling was examined using generalized estimating equations. Sources of potential bias identified using DAGs were addressed.

Results

Among male children with autism, 52.2% of next-born siblings were brothers, versus 51.0% for unaffected males. For females with autism, 50.2% of following siblings were brothers versus 51.2% among control females. The relative risk of a subsequent male sibling associated with autism diagnosis was 1.02 (95% confidence interval: 0.99, 1.04).

Conclusions

In a large, population-based sample we failed to find evidence suggesting an excess of brothers among children with autism while controlling for several threats to validity. This test cannot rule out a role of any given exposure, including prenatal testosterone, in either risk of autism or offspring sex ratio, but suggests against a common cause of both.     

Birth interval, mortality and growth of children in a rural area in Kenya

The impact of the length of birth intervals on mortality and growth of children from the perinatal period to 2 years in the Northern Division of Machakos District, Eastern Province, Kenya, were analyzed. There are 2 types of birth intervals: 1) the prospective birth interval--between the birth concerned (the 1st birth of the interval pair) and the subsequent birth; and 2) the retrospective birth interval--between the birth considered (the 2nd of the interval pair) and the preceeding birth. This study includes 3019 women who had at least 1 live birth between April, 1974 and April, 1981. They gave birth to 6778 children (including stillbirths). Births occurring in 1974 are excluded in the analysis because of considerable underregistration. 102 stillbirths and 213 deaths in the 1st 2 years are analyzed. They have been grouped into deaths during the perimatal period; the 1st year after the 1st week of life (infant period); and the 2nd year of life. The most convient method of analysis of the relation between retrospective birth interval and mortality is multivariate analysis, as the intermedicate biological and behavioral factors through which birth intervals can affect health are simultaneously influenced by other variables like maternal age and birth order; the log linear model is applied here. The probability of dying is the dependent variable. The impact of short prospective intervals are closely associated. Only infant and child deaths occurring after the conception of the next child are included. The size of cohorts in which these deaths occur can be calculated with a life table approach. The mortality probability between 5 and 12 months for children with short prospective intervals is .034. This is higher than the corresponding rate for all children in the area (P0.05). It is shown that children with short retrospective or prospective birth intervals do not run a greater risk of mortality or growth retardation than children with longer intervals, neither during the perinatal period nor during the 1st 2 years of life.     

Mortality rates in subjects with fetal alcohol spectrum disorders and their siblings

BACKGROUND: Our objective was to estimate the mortality rate in subjects with fetal alcohol spectrum disorders (FASD) and their siblings whose FASD status was unknown. METHODS: We used the state FASD Registry to link subjects with FASD to a North Dakota birth certificate. We were able to link 304 of 486 cases (63%). We used the birth certificates to identify the mother and children born to the mother (siblings). We then searched for death certificates for both the FASD cases and their siblings. We then calculated the annual and age‐adjusted mortality rates for the siblings of the Registry cases and compared them with mortality rates from North Dakota. RESULTS: The FASD case mortality rate was 2.4%, with a 4.5% mortality rate for their sibings, accounting for 14% of all deaths when compared to the North Dakota residents matched by age and year of death. The sibling deaths accounted for 21.5% of all cause mortality matched by age and year of death. The age‐standardized mortality ratios were 4.9 for the FASD cases and 2.6 for their siblings whose FASD status was unknown. CONCLUSIONS: Mortality rates for FASD cases and their siblings were increased and represent a substantial proportion of all cause mortality in North Dakota. Prevention of FASD may be a useful strategy to decrease mortality. Birth Defects Research (Part A), 2008. © 2008 Wiley‐Liss, Inc.     

Mother-child separation: a study in a remote health care setting.

The literature indicates that the birth of a sibling and the consequent temporary separation from the mother is usually a stressful experience for a child. It was hypothesized that this stress would result in an increased number of visits by the child to health care facilities because of new health problems. In a controlled study of 89 matched pairs of Indian families in a remote region of northwestern Ontario this hypothesis was not supported. During the intervals studied - the anticipatory period before delivery, the separation itself and the period immediately following the mother''s return home - the number of diagnoses of new medical problems was significantly less for the children who were separated from their mothers for the birth of a sibling. As well, the number of diagnoses of new medical problems in the children separated form their mothers decreased over the three intervals. The fathers'' reluctance to seek health care probably played a major role in this decrease.     

Netherlands Twin Register: a focus on longitudinal research.

In 1986 we began The Netherlands Twin Register (NTR) by recruiting young twins and multiples a few weeks or months after birth. Currently we register around 50% of all newborn multiples in The Netherlands. Their parents receive a questionnaire at registration and afterwards when the children are 2, 3, 5, 7, 10 and 12 years of age. Teachers are asked to rate the behavior of the children at ages 7, 10 and 12 years. Adolescent and young-adult twins were recruited through City Councils in the early 1990s. These twins, their parents and siblings participate in longitudinal survey studies that include items about health, fertility, lifestyle, addiction, personality and psychopathology, religion, socioeconomic status, and educational attainment. The total number of twins and multiples registered with the NTR is currently over 60,000. Subgroups of twins and siblings take part in studies of cognitive development, brain function and neuropsychological indices of attention processes, and molecular genetic studies of classical and behavioral cardiovascular risk factors. DNA samples are currently collected in selected twin families for two large linkage studies, which aim to find QTLs for anxious depression and for nicotine addiction. Sisters who are mothers of DZ twins contribute DNA samples for a linkage study of DZ twinning. Large cohorts of phenotyped family members from the general population are very valuable for genetic epidemiological studies and permit selection of informative families for gene finding studies.     

Unemployment and child abuse.

The employment state of men living in the homes of children at the time that child abuse was diagnosed was determined. The series included a wide range of abuse, including non-accidental injury, failure to thrive, neglect, and emotional deprivation. Two cohorts of children seen during 1974-9 and 1980-5 were compared; these periods were chosen because a large increase in unemployment began in Sheffield in 1980. Although the proportion of the men without work was significantly increased during the second period, this increase could not be ascribed to the rise in either long term or short term unemployment among those who had previously been in regular employment. It was accounted for by a rise in the proportions of single parent families and families in which the resident man had never had regular employment. This may reflect an increase in pregnancies among young mothers. There was no evidence to support the belief that the loss of a job in otherwise stable families leads to an increase in child abuse.     

A dynamic framework for the study of optimal birth intervals reveals the importance of sibling competition and mortality risks

Human reproductive patterns have been well studied, but the mechanisms by which physiology, ecology and existing kin interact to affect the life history need quantification. Here, we create a model to investigate how age‐specific interbirth intervals adapt to environmental and intrinsic mortality, and how birth patterns can be shaped by competition and help between siblings. The model provides a flexible framework for studying the processes underlying human reproductive scheduling. We developed a state‐based optimality model to determine age‐dependent and family‐dependent sets of reproductive strategies, including the state of the mother and her offspring. We parameterized the model with realistic mortality curves derived from five human populations. Overall, optimal birth intervals increase until the age of 30 after which they remain relatively constant until the end of the reproductive lifespan. Offspring helping each other does not have much effect on birth intervals. Increasing infant and senescent mortality in different populations decreases interbirth intervals. We show that sibling competition and infant mortality interact to lengthen interbirth intervals. In lower‐mortality populations, intense sibling competition pushes births further apart. Varying the adult risk of mortality alone has no effect on birth intervals between populations; competition between offspring drives the differences in birth intervals only when infant mortality is low. These results are relevant to understanding the demographic transition, because our model predicts that sibling competition becomes an important determinant of optimal interbirth intervals only when mortality is low, as in post‐transition societies. We do not predict that these effects alone can select for menopause.     

Variations in value orientations and fertility behavior

A test of a hypothesis regarding attitudes toward nature, time, familial relations, activity, and sex was undertaken. The hypothesis was: wives who have a modern orientation to any 1 of these items will expect a smaller number of children, will have smaller proportion of unplanned births, and will have lower fertility than wives who have traditional orientation. Data on which the study was based were obtained from a project on family formation and values conducted in Lexington, Kentucky, during the spring of 1968. Of the 403 eligible respondents (resident white nonfarm females who had a legitimate live birth in Fayette County, Kentucky during the period January 1, 1967, to December 31, 1967, 275 were interviewed. Results indicated 3 patterns: 1) sex orientation is not related to fertility expectations, or, at best, is weakly associated; 2) the sex orientation scale does not discriminate birth control effectiveness as predicted; and 3) sex orientation is associated with actual fertility, but in the opposite direction from that hypothesized. Women who have a modern orientation to sex have larger families than women with a traditional orientation, even though the former are more effective family planners. Results indicated that time orientation is not directly related to fertility behavior while the activity orientation is related to fertility behavior but in the opposite direction from that hypothesized. The nature and relational orientations provided some support for the hypothesized relation between modern orientations and low fertility behavior.     

A Cultural–Historical Perspective on the Depressed Fertility Among the Matrilineal Moso in Southwest China

Family systems exert substantial influence on the demographic processes of populations. The Moso in Southwest China feature an institutionalized visiting sexual system, which differs from marriage, and a grand household system without conjugal units. Moso women enjoy virtually unlimited reproductive autonomy. On the basis of surveys of 127 Moso households conducted in 1988 and 1989, this paper analyzes the fertility experience of Moso women during the twentieth century and the mortality rates of the Moso population in the PRC era. Moso fertility patterns are characterized by a late age at first birth (median 23 years), long interbirth intervals (median 3 years), and a high rate of childlessness (16%). We conclude that this pattern of low fertility is an outcome of the unique Moso cultural practices. For the cohorts of Moso women born between 1905 and 1929, pathological sterility caused by STD is likely to have depressed fertility as well.