Only child benefits – show me the research!

Personality traits

Only children comparable to others in most personality respects; score higher on achievement motivation and personal adjustment

A quantitative review of the literature on the personality characteristics of only children was conducted to provide a baseline, archival resource on 16 different personality domains and to advance theory in the area of family configuration effects on personality development. This review combined the results of 141 studies and found that only children scored significantly better than other groups in achievement motivationand personal adjustment. The achievement motivation finding was especially reliable, persisting across several comparison groups. Overall, however, the review indicated that only children were comparable in most respects to their siblinged counterparts. The findings are discussed in terms of parent-child relations and socioeconomic factors.

http://www.jstor.org/stable/352302?seq=1#page_scan_tab_contents

Only children are similar in development to first borns and kids from two-child families

Results indicated that developmental outcomes of only children were similar to outcomes for firstborns and children from two-child families but dissimilar to outcomes for later borns and children from larger families. Furthermore, outcomes were more positive for only children, firstborns, and children from two-child families than for all other comparison groups. In agreement with Falbo and Polit, only-childdeprivation and only-childuniqueness explanations for outcome differences were not supported in favor of an explanation that emphasizes qualities of the parent-child relationship.

https://www.ncbi.nlm.nih.gov/pubmed/2388055

As adults, only children are more ambitious, confident, intelligent and independent

The purpose of this study was to determine how birth position i.e. first-born, middle-born, lastborn, and only child, correlates with personality. MATERIAL AND METHOD: One hundred and eighty from 186 (97%) 1st year medical students of Chiang Mai Medical School [Thailand] were asked to complete a questionnaire and take a personality test. The data obtained included age, sex, Grade Point Average (GPA), and family background i.e. birth order and the students’ personality profiles, which were assessed by the California Psychological Inventory. RESULTS: The results showed that only children and lastborn had more positive personality factors than other groups. They had more ambition, breadth of interests, versatility, self confidence, clear-thinking, intelligence, and independencethan first-born and middle-born (Cs and Ai scale on CPI).

https://www.ncbi.nlm.nih.gov/pubmed/12075729

Only children surpass first borns in character and positive relationship with parents

Conducted 6 meta-analyses of 115 studies on only children to evaluate the status of OCs. The meta-analyses focused on achievement, adjustment, character, intelligence, parent–child relationships, and sociability. Findings indicate that only children were found to surpass all others except firstborns and people from 2-child families on achievementand intelligence.They also surpassed all non-only children, especially people from families with 3 or more children, in characterand they surpassed all non-only children, especially those from large families, in the positivity of the parent–child relationship. Across all developmental outcomes, only children were indistinguishable from firstborns and people from small families. Theories relating to only children deprivation and only child uniqueness were discredited by the results of the meta-analyses. The meta-analyses supported parent–child relationships as an important factor in producing the developmental outcomes attained by only children, firstborns, and people from 2-child families. Studies included in the meta-analyses are appended.

http://psycnet.apa.org/psycinfo/1987-00682-001

Large study finds personality not related to being an only child

Birth-order effects on traits within the five-factor model of personality were examined in three studies. Self-reports on brief measures of Neuroticism, Extraversion, and Opennessin a national sample (N= 9664) were unrelated to birth order. Self-reports on the 30 facet scales of the Revised NEO Personality Inventory (NEO-PI-R) in an adult sample (N= 612) showed only small effects for Altruismand Tender-Mindedness. Peer ratings (N= 166) supported the hypotheses that laterborn children would be higher in facets of Opennessand Agreeableness, but spouse ratings (N= 88) did not replicate those findings. Birth order may have subtle effects on perceived personality, but it is unlikely that this effect mediates associations with scientific radicalism.

https://www.sciencedirect.com/science/article/abs/pii/S0092656698922334?via%3Dihub

Only children from middle class families, but not upper class, have higher achievement motivation

In an investigation of some demographic correlates of achievement motivation, projective tests and questionnaires were administered to two samples of boys between 8 and 14 years of age. The following results were obtained: In the upper classes medium-size families produced boys with the highest scores, while in the middle class the smaller the family the larger the score. In both samples the large size families in Classes I-II and V have the lowest scores of all groups. The sons of young mothers had higher achievement motivation than the sons of older mothers only when family size was small.

http://psycnet.apa.org/record/1962-04610-001

Only children feel just as happy and popular and have similar self-esteem; however, they don’t feel as good at sports

The present study considers adolescent singletons in the Netherlands. It examines the related claims that onlychildren have a less happy youth because they are pressed into adult thinking and behavior too early and that they stand out as “little eggheads”–good at school, but not very sportsmanlike, and unpopular among their peers. Data were gathered by means of questionnaires administered to 2,511 secondary schoolchildren. The onlychildren in this sample neither appeared to be less happy nor was their global self-esteem any lower. The “little egghead” hypothesis was onlypartly confirmed. Onlychildren feel themselves to be less proficient in sports. However, they do not consider themselves better in school or less popular among peers.

https://www.ncbi.nlm.nih.gov/pubmed/2728965

As adults, only children are more autonomous, but not more selfish or lacking in social skills

Two studies are reported: one examines stereotypes about only children and the other examines stereotypes about mothers of only children. A sample of 150 college students were the subjects for the first study which utilized the Prisoner’s Dilemma Game, the NASA exercise, and a questionnaire to test the hypotheses that only children are selfish, lack social skills, are autonomous and suffer as a result of being only children. The only hypothesis that held up was that only children appear to be more autonomous. The second study interviewed 76 mothers of college undergraduates through a mail survey, providing information as to why mothers of only children have one child. Answers were given relevant to age, education, birth complications, and folklore beliefs. (MS)

https://www.researchgate.net/publication/234579390_Folklore_and_the_Only_Child_A_Reassessment

Only children are more likely to be submissive and have overprotective parents

This study was undertaken to provide an evaluation of the mental health problems associated with the only childand a basis for further studies of singletons. Data were obtained from the recores of the Mental Health Clinic, London Psychiatric Hospital, of 115 only children and 115 non-only children between 5 and 15 years of age. The results showed that the only childdid not differ significantly from his non-only peer in intelligence, school progress, referral reasons and the majority of mental health disturbances, but a significant difference was found in his increased submissiveness, greater number of repeat visits to clinic, and in the overprotective attitude of his parents. Aggressive behaviour was more frequently observed in the control group, which also had a significantly higher proportion of rejecting mothers. Suggestions for further research in this area are made.

https://www.ncbi.nlm.nih.gov/pubmed/1125931

The purpose of this study was to explore the differences of character strengths and their predictors between only-childand non-only-childnursing students. Using data from 742 participants, linear regression was conducted. Results indicate that there was no significant differences between the two groups concerning anxiety, depression and general self-efficacy. The character strengthsstatus of only-childwas better than that of non-only-child and the predictors of character strengths between the two groups had some differences. This study has suggestions on how to help nursing students to improve their character strengths.

https://www.ncbi.nlm.nih.gov/pubmed/26577548

Only children are more creative but less agreeable

There are few studies that have focused on the topic of whether different family environments influence children’s brain structural development. Thus, in the present study, we investigated the differences in cognition (e.g., intelligence and creativity) and personality and the anatomical structural differences of gray matter volume (GMV) between only-children and non-only-children. The behavioral results revealed that only-children exhibited higher flexibilityscores (a dimension of creativity) and lower agreeablenessscores (a dimension of personality traits) than non-only-children. Most importantly, there were significant differences in the gray matter volume between only-children and non-only-children that occurred mainly in the brain regions of the supramarginal gyrus, which was positively correlated with flexibility scores; the medial prefrontal cortex, which was positively correlated with agreeableness scores; and the parahippocampal gyrus.

https://www.ncbi.nlm.nih.gov/pubmed/26956710

Intelligence of only children

First borns and only children have higher IQ; effect seen in older children

A sample of around 2,500 adolescents in a child and adolescent psychiatry clinic in the region of Münster, Germany had their intelligence assessed. Family size (total number of siblings within a family) was significantly correlated with intelligence score categories (—0.08 and —0.19 for males and females). First borns and only children displayed higher IQs than later borns. The relationship was found only for those older children (11 or older) and not the under 11-year-olds. The relationship between birth order and intelligence was moderated by gender.

https://journals.sagepub.com/doi/10.1177/0143034308101849

Birth order no longer affects IQ as children get older

Longitudinal IQ data were available from 626 Dutch twin pairs at ages 5, 12 and 18 years. IQ data were analyzed with a model in which age cohort, number of older sibs, sex and all interactions were included as fixed effects. The correlation across time between IQ scores was modeled a function of genetic and environmental factors. Heritability for IQ was estimated at 37% at age 5 (common environment explained 34% of the variance). At ages 12 and 18 heritability for IQ was 81% and 82%, respectively. At these ages common environment did not contribute to IQ variation.

http://psycnet.apa.org/record/2008-15166-014

Only children are more likely to have language impairment

A prospective descriptive study conducted with 170 children and their parents assisted at a school clinic in the period between March 2010 and July 2012. RESULTS: The demographic profile is composed of male children aged between 4 and 5 years old. The factors related to family and considered as risks for language impairment were being an only childand having a family history of speech and language disorders. As for the children’ s health, prematurity, hospitalization for a long period, and the presence of deleterious oral habits were also considered as risk factors.

https://www.ncbi.nlm.nih.gov/pubmed/24408550

Education

Only children more likely to excel in education, occupation as adults; no difference seen in happiness or life satisfaction

Data were obtained from a sample of 537 white, intact, married couples residing in middle to upper-middle class communities near Boston. In the sample, 70 wives and 62 husbands were only children. Compared with other first borns with siblings, and with individuals of higher birth orders, only children were found to have higher educational levels, higher occupational status, smaller families, and to be more secularly oriented. Female onlies were more likely to be working, to have planned their families before marriage, and to have been more autonomous in deciding to work. The three groups did not differ in terms of perceived happiness or satisfaction with life. They were also similar in their social activities and in the ways their children viewed them as parents. The data thus do not support the notion that only children are emotionally or personally handicapped by their lack of siblings.

http://www.jstor.org/stable/583722?seq=1#page_scan_tab_contents

Only children spend more time on homework and have better grades than those from large families

This study uses a large and representative sample of adolescents to test the theoretically informed hypotheses comparing adolescent singletons with those who have siblings. The results found that, for academic related outcomes (educational expectations, time spent on homework, and self-reported grades), there are no differences between singletons and firstborns who have any number of younger siblings. Singletons are also not different from laterborns from two-child families. In contrast, singletons are more advantageous compared to laterborns who have two or more siblings on educational expectations and grades. Singletons also spend more time on homework than laterborns who have three or more siblings. For psychosocial outcomes (psychological distress, susceptibility to negative peer pressure, and problem behaviors), singletons are not different from both firstborns and laterborns with any number of siblings. The findings suggest that singletons are not at any disadvantage compared to their peers who have siblings and they enjoy some advantages over laterborns from medium to large families on academic related outcomes.

https://www.hindawi.com/journals/cdr/2014/578289/abs/

As teens, only children are better adjusted and more likely to anticipate going to college

Chinese singleton adolescents are more likely to anticipate going to college than non-singleton adolescents. Further, singletons are more associated with conventional peers and they report better adjustments both psychologically and behaviorallythan non-singleton adolescents. Singletons and non-singletons, however, are not different in their self-reported performance in four school subjects, namely, Chinese, Math, English, and Political Studies.

https://www.ncbi.nlm.nih.gov/pubmed/19651433

Socialization

Only children are less liked by classmates; more likely to be victims or aggressors

In this study, we tested the assumption that having a sibling provides practice with skills that generalize to peer relations, by comparing the peer-related social competence of only children, first-borns with one sibling, and second-borns with one sibling in a sample of 139 elementary school-age children. Only children were similar to classmates in terms of number of close friendships and friendship quality, but were less liked by classmates as a group. Only children were more likely both to be victimized and aggressive in the peer group, suggesting that having a sibling may be especially helpful for learning to manage conflict. Results are discussed in terms of the need to examine multiple levels of social complexity to understand family-peer links.

http://spr.sagepub.com/content/19/3/299.short

Only children less cooperative, persistent and well liked by peers

993 rural and urban kindergarten and primary school Chinese children (aged 4–6 and 9–10 yrs) were rated on 7 behavioral qualities, including independent thinking, persistence, behavior control, frustration proneness, cooperation, peer prestige, and egocentrism. Results, based on 180 matched pairs of only and sibling children, revealed that only children were more egocentric, whereas sibling children possessed the positive qualities of persistence, cooperation, and peer prestige. Occupation and educational background of parents and the number of generations living together were not decisive in determining Ss’ behavioral qualities. It is concluded that cooperation and peer prestige are enhanced by the presence of siblings in the family.

http://psycnet.apa.org/psycinfo/1986-21663-001

Only children are less likely to be bullied and more likely to confide in parents

A cross sectional survey was carried out using self completion questionnaires, anthropometry, and haemoglobin measurement in middle schools (predominant age 12-16 years) in three distinct socioeconomic areas of Zhejiang province, eastern China. Data were obtained for 4197 participants. No significant differences were found between only children and those with siblings for some key indicators: underweight 19% v 18%, suicide ideation 14% v 14%, and ever smoking 17% v 15%. Only children were more likely to be overweight (4.8% v 1.5%), and to have attended a doctor (71% v 63%) or dentist (17% v 10%) in the past year. Sibling children are significantly more likely to be anaemic (42% v 32%) and to admit to depression (41% v 21%) or anxiety (45% v 37%). However, after adjusting for area, sex, and parental education levels only two differences remained: sibling children are more likely to be bullied (odds ratio 1.5) and are less likely to confide in parents (odds ratio 0.6). There were no significant differences in the key parameters between first and second born children. CONCLUSIONS: We found no detrimental effects of being an only childusing the indicators measured. Being an only childmay confer some benefits, particularly in terms of socialisation.

https://www.ncbi.nlm.nih.gov/pubmed/12765907

Teenage only children less likely to experience alcohol intoxication than first borns

This study encompassed 8% of only children, 35% first-born, and 57% non-first born adolescents of a socioeconomic homogeneous sample. Social behavior, parents and peer relationships, sports participation, smoking and illicit drug were not associated with birth order. Only children were less likely to report an episode of alcohol intoxication (39%) than first-born (68.9%) and adolescents with siblings (72.3%). Only children had high school achievement than adolescents with siblings. Sexual behavior distinguished only children due to younger age at first sexual intercourse and lower rate of heterosexual self-identification, which persisted even after adjustment for confounding variables in comparison with non first-born adolescents.

https://www.ncbi.nlm.nih.gov/pubmed/15057835

Adult only children do not differ in social skills or social competence

Differences in personality and social skills of 146 adults with 1 or more siblings and 51 adults without siblings were investigated to examine the importance of family structure for individual personality development. Participants completed the Social Skills Inventory, which measures social and emotional sensitivity, expressivity, and control, and the Eysenck Personality Questionnaire, which measures the personality factors of Extraversion-Introversion, Neuroticism-Stability, and Psychoticism. Results revealed only a marginal difference between individuals with and without siblings in Neuroticism, and no differences in various social skills or overall social competence. Possible explanations for the lack of differences in social skills between adults with and without siblings are discussed.

https://www.ncbi.nlm.nih.gov/pubmed/10507140

As adults, only children more likely to cooperate; report having fewer friends overall but similar number of close friends

A study was conducted to investigate the validity of many of the stereotypes about only children. A sample of 77 undergraduates (30 only, 30 first, and 17 last born) underwent a two-play Prisoner’s Dilemma Game, the NASA exercise, and completed a questionnaire. Results indicated that only children were more likely to make cooperative response moves in the Prisoner’s Dilemma Game and deviated more from the group decision in the NASA exercise than nononlies. Only children reported having fewer friendsand joining fewer clubs than nononlies, but reported comparable numbers of close friendsand took a comparable number of leadership positions in the clubs they joined. Results are discussed in terms of effects of siblings on development of interpersonal behavior.

https://www.researchgate.net/publication/230107164_Only_Children_and_Interpersonal_Behavior_An_Experimental_and_Survey_Study

Female only children are more comfortable with sex, less likely to experience coercion

Of 4,769 female undergraduate students, 41.0% were only-childand 59.0% were students with siblings. Compared with students with siblings, only-childstudents scored higher on sex-related knowledge, were more inclined to agree with premarital sex, multiple sex partners, one-night stands, extramarital lovers and homosexuality, and were more likely to have a boyfriend and experience sexual intercourse (73.6% vs. 61.4%; 24.0% vs. 14.0%). Only-children were less likely to experience coercion at first sex and have first sexual intercourse with men not their “boyfriends” than children with siblings (3.3% vs. 6.4%; 20.7% vs. 28.8%). There were no significant differences on other risky sexual behaviors (e.g. multiple sex partners and inconsistent condom use) between the only-childstudents and students with siblings.

https://www.ncbi.nlm.nih.gov/pubmed/24023905

Male only children, but not female, are three times more likely to divorce; male firstborns also at higher risk

A man who grew up as an only childwas almost three times as likely to divorce compared to a man with siblings, and this association was highly significant. There was no such evidence with respect to women. After controlling for sibship size, earlier born men–but not women–appeared to be at higher risk for divorce compared to those later born. There was no evidence that the gender structure of the sibship substantially affects the risk for divorce.

https://www.ncbi.nlm.nih.gov/pubmed/10897497

Family structure

60 women and 60 men between the ages of 18 and 45 years (M = 30.5, SD = 9.6) were categorized by sex, age, and birth order (only child, firstborn, lastborn) to assess the differences among the adult only-child, the youngest child, and the oldest child in autonomous characteristics and cohesiveness in family interaction. Analysis of the responses on a biographical data sheet, the California Psychological Inventory, and the Family Adaptability Cohesion Scales III showed that main effects for birth order and sex are significant in the process of separation-individuation and that the only childis less autonomous than the oldest child.

https://www.ncbi.nlm.nih.gov/pubmed/8367556

As adults, only children see relatives 20% less often than those with siblings

We find that respondents with siblings see relatives more often than do only children, on average just over 40 times per year compared to 32 times. However, other studies report increased visits with parents for adult only children, particularly women. Socializing with neighbors (on average around 30 times per year), with coworkers (around 16 times per year) and with friends who are not neighbors (34 times per year) occur with the same frequency for adult only children and respondents with siblings. We also find that at younger adult ages growing up without siblings affects the frequency of engaging in certain social events – less time spent with friends, more time spent at church/synagogue, literary/art or professional/academic events – but these differences decrease with age. The larger difference observed during the young adult years is reduced to a small difference by age 40 (less than 3 social activities per year) and there is no difference by age 50.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3237053/

Positive sibling relationships may decrease loneliness in old age

This paper reports the findings of a qualitative examination of sibling relationships in old age. Interviews with sixty people over the age of sixty-five revealed that interactions with sisters and brothers took on new meaning in late life. A shared history of lifetime experiences made the sibling relationship unique in social networks in old age. Those who had positive relationships with siblings found that interactions decreased feelings of loneliness, provided emotional support and validation of earlier life experiences, and built feelings of closeness and sibling solidarity. Even those who had negative sibling relationships indicated a shift in feelings. The intensity of feeling about siblings in old age suggests that further study of the later-life sibling bond might increase understanding of ways in which the social and emotional needs of older people can be met. Interviews provided an effective method for gathering rich data about these complex social and emotional interactions.

https://www.cambridge.org/core/journals/canadian-journal-on-aging-la-revue-canadienne-du-vieillissement/article/siblings-in-old-age-something-special/B3FD670BF602F6BD3B83015CA70EAD84

66% of elderly with one sibling regard them as close friend

http://journals.sagepub.com/doi/pdf/10.1177/0002764289033001008

Lack of sibling interaction does not affect morale in elderly

Previous research has found no relationship between the morale of the elderly and frequency of interaction with children. This is often explained in terms of cohort differences in attitudes, values, and interests between parents and children. This study examines the relationship between morale and interaction with siblings among the elderly and finds no relationship. The indicated conclusion is that, at least in terms of interaction frequency, relations with kin do not affect the morale of the elderly. Furthermore, age differences are not relevant to the explanation of this fact.

http://journals.sagepub.com/doi/10.1177/016402758023005

Only children do not spend more hours caring for aging parents

Based on matched data from the 1982 National Long-Term Care Survey and the National Survey of Informal Caregivers, this article explores the degree to which separating adult children (N = 3,742) by the composition of their sibling network (i.e., only children, single-gender networks, and mixed-gender networks) provides insight into the association between gender and patterns of parent-care. The data reveal that, within all sibling network categories, daughters were more likely than sons to be providing care to an impaired parent; however, the repercussions of being a caregiver were not similarly uniform. Specifically, sons and daughters from only-childand single-gender networks reported a similar number of hours per day spent in parent-care and experienced comparable levels of stress and burden. In contrast, daughters from mixed-gender networks reported significantly higher levels of stress and burden and more hours per day spent caregiving than sons.

https://www.ncbi.nlm.nih.gov/pubmed/2359872

Psychological problems

Those with four or more siblings have more internalizing problems, worse self-concept than only children

The results suggest that sibship size has only a modest effect on personality traits among early adolescents. Specifically, only those adolescents who have at least four siblings are found to have significantly worse internalizing problem behaviors, worse self-concept, and worse locus of control compared to only children. In addition, this study finds little evidence that adolescents benefit more from sisters than brothers. Lastly, compared to having older siblings, having younger siblings is more beneficial for personality traits in predicting self-concept and locus of control.

https://www.sciencedirect.com/science/article/abs/pii/S0362331913000918

More narcissistic disorder in only children, first borns

To study the relationship between birth order and pathological narcissism, it was predicted that firstborn and only children would score significantly higher on standardized measures of pathological narcissism. Two such measures, the Millon Clinical Multiaxial Inventory and the Narcissistic Personality Inventory, were administered to 50 randomly selected subjects from a metropolitan mental health and family treatment agency. Subjects were asked to indicate their ordinal birth positions, e.g., first, middle, last, or only, and then were administered both instruments. Analysis supported the initial prediction by indicating that firstborn and only children had higher mean scores on the measures of pathological narcissism. It might be advisable for clinicians to identify patients’ ordinal positions while appraising relevant diagnostic criteria and eventual treatment planning.

https://journals.sagepub.com/doi/abs/10.2466/pr0.1993.72.1.311

Narcissism is not associated with being an only child

The purpose of this investigation was to clarify the relationship between birth-orderposition and the development of narcissism, while refining research and theory. The relationship between birth-orderstatus and narcissismwas examined with a sample of 79 undergraduate students (55 women and 24 men). These subjects were placed in one of the four following birth-ordercategories of firstborn, second-born, last-born, and only children. These categories were chosen given their significance in Adlerian theory. Each subject completed the Narcissistic Personality Inventory and a demographic inventory. Based on psychodynamic theory, it was hypothesized that firstborn children were expected to score highest, but statistical significance was not found for an association between narcissismand birth order. Further research is urged to investigate personality theory as it relates to parenting style and birth order.

https://www.ncbi.nlm.nih.gov/pubmed/9148293

Only children are not more neurotic as adults

The authors compared the frequency of the positions among siblings in the neurotic and the control group. The investigation was performed on 582 persons, 435 of them had undergone treatment in the psychotherapeutic department for neuroses. The rest of them, the control group, had never been treated for neurosis. It does not seem that the position of the oldest, the youngest and the only childleads to neurotic disorders. The most risky position is evident by the middle sister. However, validity of these findings is limited by methodological reasons.

https://www.ncbi.nlm.nih.gov/pubmed/8004683

Only children are more likely to suffer from schizophrenia

Possibilities, difficulties and results of statistic investigations on the relation between schizophrenia, depression, neurosis and alcoholism are reported: The schizophrenic patients showed an excess of single childs and children of large sibship. In depressive patients the eldest are overrepresented. In neurotics single and eldest children are more numerous than in the general population. In alcoholic males the last born und specially the youngest children are overrepresented. But nearly all of the relations found here have been contested by some other authors.

https://www.ncbi.nlm.nih.gov/pubmed/583869

Only children are at greater risk than first borns of having psychiatric disorder

Data were obtained from the preinduction examination records of approximately 400,000 Dutch males who had been administered a battery of psychometric tests (Raven Progressive Matrices, a test of arithmetic and mathematics achievement, a test of grammar and language achievement, a clerical aptitude test, and the Bennett Test of Mechanical Comprehension). Results show the following: (a) Birth order was related to intellectual performance, and this effect was present even when family size and social class were controlled. (b) The larger the family size, the higher the school failure rate. (c) Height and IQ score both showed a family size effect—members of smaller families were taller and had better IQ scores. (d) Only and last born children were at greater risk of being diagnosed as psychiatrically disordered than were firstborns.

http://psycnet.apa.org/record/1979-13439-001

Only children are more likely to suffer from psychiatric disorders in adulthood

This study is a substudy of the prospective Northern Finland 1966 Birth Cohort Project with 1588 young adult subjects. RESULTS: After adjusting for confounders (gender, parental social class and parental psychiatric disorder) the results indicated that being an only childin childhood was associated with cluster A psychiatric disorders [Paranoid, Schizoid, and Schizotypal Personality Disorders]. CONCLUSIONS: Results suggest that single-parent family type at birth and being an only childin the 1960s are associated with psychiatric disorders in adulthood.

https://www.ncbi.nlm.nih.gov/pubmed/18328677

As adults, male first borns also have higher risk of schizophrenia

The Northern Finland 1966 Birth Cohort was studied in order to investigate the association between birth order and schizophrenia. METHOD: Effects were adjusted for wantedness of pregnancy, perinatal complications, maternal age at delivery, family type and number of siblings. RESULTS: The risk for schizophrenia was elevated among male first-borns (ratio 1.5) and female last-borns (ratio 1.3). The risk was lower than expected among male last-borns (ratio 0.7) and females belonging to other status (ratio 0.6).

https://www.ncbi.nlm.nih.gov/pubmed/11473509

Male only children more likely to commit violent crime

Serious defects in social skills acquired during childhood may be associated with aggressive behavior in later life. The authors studied whether being an only childwas associated with criminality in adulthood and, secondly, if parental factors increased the putative risk. RESULTS: The risk for violent crimes later in life was elevated among the only children. If perinatal or parental risks were combined with being an only child, the odds ratios for violent offending increased four-fold to eight-fold. A corresponding risk increase between being an only childand nonviolent offending was not detected. CONCLUSIONS: These results support the hypothesis that growing up as an only childis associated with violent criminality among male subjects.

https://www.ncbi.nlm.nih.gov/pubmed/11384909https://www.ncbi.nlm.nih.gov/pubmed/11384909

Having siblings associated with negative effects on psychological well-being in adults

Data from seven U. S. national surveys were used to estimate the effects of number of siblings on eight dimensions of psychological well-being among adult white males and white females. All of the statistically significant estimated effects of having siblings are negative, and most of these cannot be accounted for by the lower mean family incomes and amount of education of the persons with siblings. The evidence for negative effects is stronger for white males than for white females. The evidence from this and previous studies is generally inconsistent with the popular stereotype of the unhappy, maladjusted only child, but additional evidence is needed before the issue is considered closed.

http://jfi.sagepub.com/content/5/3/363.short

Young only children are no more likely to have psychiatric problems

We gathered data from 169 children under age 5 years seen in the psychiatric department of a large pediatric hospital in Montreal, Quebec. RESULTS: First, bivariate analysis showed no differences between the proportion of only children and children with 1 sibling regarding absence of a psychiatric diagnosis. Second, multivariate logistic regression analysis revealed that child’s age and mother’s child-rearing attitudes were significant variables.

https://www.ncbi.nlm.nih.gov/pubmed/15147026

Only children more likely to have internalizing problems; kids with siblings more likely to have externalizing problems; neither more likely to have problems, overall

These findings illustrate that, among boys, children without siblings tended to have ‘internalizing’ behavior problems, children with siblings tended to have ‘externalizing’ behavior problems at the preschool age. Female children without siblings began to have ‘externalizing’ behavior problems when they reached early school ages of 7-9 years, but then reverted to ‘internalizing’ behavior problems when they reached the late school ages of 9-11 years.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1440-1819.1998.tb03223.x

Obesity