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In a sample of 273 American college students who were administered seven personality tests, only death obsession scores were consistently associated with sibship size and birth order (not optimism, pessimism, anxiety, a Taoist orientation, suicidal ideation, or obsessive-compulsive tendencies).
"Nutrients available to children are determined largely by intrahousehold allocations. There are a number of reasons why birth order may affect these allocations. A model is developed to estimate critical parameters of parental preferences regarding the allocation of nutrients among their children. Latent variable estimates for rural south India indicate that parental preferences have productivity-equity tradeoffs and parents favor older children. The productivity-equity tradeoff, however, is much less for the lean season. Therefore, when food is scarcest, parents follow more closely a pure investment strategy, exposing their more vulnerable children to greater malnutrition risk."
The purpose of the study was to determine whether the incidence of periventricular-intraventricular hemorrhage (PV-IVH) is significantly different between the firstborn and secondborn of very low birth weight twins. The firstborn and secondborn of 102 sets of twins were compared by incidence of PV-IVH, mode of delivery, birth weight, 5-minute Apgar score, pH, duration of oxygen therapy, duration of ventilator support, and occurrence of patent ductus arteriosus, air leak, bronchopulmonary dysplasia, or hyaline membrane disease. PV-IVH was graded by the Papile classification. Statistical data were analyzed by either the paired t test or McNemar's test. Major PV-IVH (grades III and IV) occurred in 11 firstborn and 14 secondborn infants. Minor PV-IVH (grades I and II) occurred in 11 firstborn and 8 secondborn twins. Neither these findings nor those from other measured variables showed a statistically significant difference.
In a study of 196 college students, those with step- and half-siblings were more depressed than those with none. Birth order and number of siblings were not associated with scores suggesting manic-depressive or suicidal tendencies.
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In the present study, 334 youths evaluated their families by responding to the Personal Attribute Inventory for Children. An analysis of variance revealed no significant main effects due to respondents' birth order or gender, but did find a significant main effect due to family structure and a significant two-way interaction effect between respondents' family structure and gender. Specifically, males from divorced remarried families and females from divorced nonremarried families were found to evaluate their respective families significantly more negatively than did their counterparts from other familial configurations. Implications of these findings are discussed.
In a random sample of 104 pairs of middle-class Caucasian, handedness-discordant twins six years of age or older, a significant relationship has been found between birth order and handedness in monozygotic twins, there being an excess of left-handed among first-born twins (P less than 0.01). No such relation has been found in dizygotic twins.
The influence of both maternal age and parity on seasonal birth patterns in Sweden was studied, using the Swedish Medical Birth Register for 1973. The register contained reports of 109 342 pregnancies. A test for seasonality, using a squared sinus function and an analysis of variance, was performed for each age and parity group. Significant differences were shown for parity irrespective of age and age groups within parity groups. There was one exception: no influence of parity could be demonstrated in women more than 35 years of age. For the lowest and the highest age groups (all parities), there was no significant seasonality in birth rate tested by the squared sinus method. This was also true for parity 3 (age group 20--24) and parity 1 (age group 30--34). The other age groups and parity classes show a significant seasonality with a maximum of births in April and May.
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Corrected registration returns in a rural area of Nigeria show that the average infant mortality rate during one five year period was 88.2 per 1000 per annum. The rate was 89.4 per 1000 for females, which did not differ significantly from that of 87.0 per 1000 for males. There were significant variations in the rates by maternal age and by birth order. Forty-two per cent of the infant deaths were of low birth-weight, the mortality rate among this group being approximately six times as high as for those with greater weights.
"This paper reports menarcheal age in a sample of 300 Andhra [India] girls with a view to contribute to the study of secular trend, and attempts to analyse the relationship of menarche to economic status of the family, diet (in broad terms), sibship size and birth order."
First pregnancies are known to have higher oestrogen levels than later ones and first-born women are at increased breast cancer risk compared with later-born women. We hypothesized that a birth order effect might be even more evident in male breast cancer patients, in whom oestrogens in adult life are generally low. In a population-based study in Denmark involving 77 male breast cancer patients and 288 population controls, first-born men compared with later-born men had a relative risk of 1.71 for the disease (95% confidence interval (CI) 1.00-2.92). This result is in line with that seen in female breast cancer cases and indicates that male breast cancer may have roots in the intrauterine life, oestrogens being a likely mediator.
The present study investigated evidence for an interaction between two of the best established etiologic factors, or markers of etiologic factors, in the literature on male homosexuality: fraternal birth order and hand preference. By combining five samples, the authors produced study groups of 1774 right-handed heterosexuals, 287 non-right-handed heterosexuals, 928 right-handed homosexuals, and 157 non-right-handed homosexuals. The results showed a significant (P = 0.004) handedness by older brothers interaction, such that (a) the typical positive correlation between homosexuality and greater numbers of older brothers holds only for right-handed males, (b) among men with no older brothers, homosexuals are more likely to be non-right-handed than heterosexuals; among men with one or more older brothers, homosexuals are less likely to be non-right-handed than heterosexuals, and (c) the odds of homosexuality are higher for men who have a non-right hand preference or who have older brothers, relative to men with neither of these features, but the odds for men with both features are similar to the odds for men with neither. These findings have at least two possible explanations: (a) the etiologic factors associated with non-right-handedness and older brothers-hypothesized to be hyperandrogenization and anti-male antibodies, respectively-counteract each other, yielding the functional equivalent of typical masculinization, and (b) the number of non-right-handed homosexuals with older brothers is smaller than expected because the combination of the older brothers factor with the non-right-handedness factor is toxic enough to lower the probability that the affected fetus will survive.
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Studies of family size in successive generations have found a small but persistently positive effect of size of family of orientation. Recent work has suggested that this relationship may be influenced by birth order, intergenerational change in lifestyle, and familial satisfaction. Data from a 24-year longitudinal study of women in Pennsylvania indicate that number of siblings does influence size of family of procreation. More important, this relationship is stronger among women who were first-born that later-born, stronger for those not experiencing intergenerational change than for those who changed, and stronger among those who at age 16 were satisfied with their parental family than for those who were dissatisfied.
In men, sexual orientation correlates with an individual's number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. It has been hypothesized that this fraternal birth order effect reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H-Y antigens) by each succeeding male fetus and the concomitantly increasing effects of such maternal immunization on the future sexual orientation of each succeeding male fetus. According to this hypothesis, anti-H-Y antibodies produced by the mother pass through the placental barrier to the fetus and affect aspects of sexual differentiation in the fetal brain. This explanation is consistent with a variety of evidence, including the apparent irrelevance of older sisters to the sexual orientation of later born males, the probable involvement of H-Y antigen in the development of sex-typical traits, and the detrimental effects of immunization of female mice to H-Y antigen on the reproductive performance of subsequent male offspring. The maternal immune hypothesis might also explain the recent finding that heterosexual males with older brothers weigh less at birth than heterosexual males with older sisters and homosexual males with older brothers weigh even less than heterosexual males with older brothers.
In Born to Rebel, F. Sulloway (1996) argued that, throughout history, later-borns have been more likely than first-borns to challenge the status quo. The authors tested Sulloway's hypothesis among a group of U.S. college students who had participated in civil disobedience as part of a labor dispute. The authors predicted that there would be a higher percentage of later-borns among those who had been arrested than among a group of their friends who had not participated in civil disobedience or among a control group of students drawn from classes at the college. The findings, in fact, revealed a significant relationship between the number of times the students had been arrested and birth order.