The confluence model: birth order as a within-family or between-family dynamic?

The confluence model explains birth-order differences in intellectual performance by quantifying the changing dynamics within the family. Wichman, Rodgers, and MacCallum (2006) claimed that these differences are a between-family phenomenon--and hence are not directly related to birth order itself. The study design and analyses presented by Wichman et al. nevertheless suffer from crucial shortcomings, including their use of unfocused tests, which cause statistically significant trends to be overlooked. In addition, Wichman et al. treated birth-order effects as a linear phenomenon thereby ignoring the confluence model's prediction that these two samples may manifest opposing results based on age. This article cites between- and within-family data that demonstrate systematic birth-order effects as predicted by the confluence model. The corpus of evidence invoked here offers strong support for the assumption of the confluence model that birth-order differences in intellectual performance are primarily a within-family phenomenon.     

Intelligence, birth order, and family size

The analysis of the National Child Development Study in the United Kingdom (n = 17,419) replicates some earlier findings and shows that genuine within-family data are not necessary to make the apparent birth-order effect on intelligence disappear. Birth order is not associated with intelligence in between-family data once the number of siblings is statistically controlled. The analyses support the admixture hypothesis, which avers that the apparent birth-order effect on intelligence is an artifact of family size, and cast doubt on the confluence and resource dilution models, both of which claim that birth order has a causal influence on children's cognitive development. The analyses suggest that birth order has no genuine causal effect on general intelligence.     

The Development of Social Facilitation of Eating in Puppies

It was hypothesized that firstborns express greater preference for reading and consultation with authorities when in need of information than middle-born or last-born adolescents. A scale was constructed which measured preferences for reading, consultation with authorities, consultation with nonauthorities, dependence upon authorities, thinking, and observation. Firstborn high school students expressed greater preference for reading for information than did middle-born and last-born students. There were no other birth-order effects.     

Birth order has no effect on intelligence: a reply and extension of previous findings

We address points raised by Zajonc and Sulloway, who reject findings showing that birth order has no effect on intelligence. Many objections to findings of null birth-order results seem to stem from a misunderstanding of the difference between study designs where birth order is confounded with true causal influences on intelligence across families and designs that control for some of these influences. We discuss some of the consequences of not appreciating the nature of this difference. When between-family confounds are controlled using appropriate study designs and techniques such as multilevel modeling, birth order is shown not to influence intelligence. We conclude with an empirical investigation of the replicability and generalizability of this approach.     

Birth-order and sex differences in the frequency of referral of children for psychological treatment: A partial replication and extension

A previous study of two-child families by Lahey, Hammer, Crumrine, and Forehand (1980) found that firstborn children were more likely to be referred for psychological treatment than second-borns and that males were more likely to be referred than females. Furthermore, there was a significant interaction between these two variables. In the present study, a new sample of two-child families from the same clinic was studied, confirming the previous findings of significant birth-order and sex effects but not the interaction. Separate analyses indicated that both birth-order and sex effects were significant for children given externalizing diagnoses, but only the birth-order effect was significant for heterogeneous internalizing diagnoses. Similarly, an analysis of only children revealed a significant sex effect for externalizers but not internalizers. A combined sample of two-child families independent of diagnosis (N=425)suggests that there are reliable associations of birth order and sex, but not the interaction, with referral rates in two-child families. Referred children were significantly more likely to have a female sibling than would be expected by chance, regardless of birth order or sex, suggesting that these children may look more deviant by comparison to female, vs. male, siblings, thus increasing the likelihood of clinic referral. The potential utility of birth order and sex as demographic marker variables for etiological research is discussed.     

The Measurement of Handedness by Preference and Performance Tests

Hand preference and hand proficiency were compared on 50 male subjects (30 self-classified left-handers and 20 self-classified right-handers) according to their responses to a 20 item questionnaire and their performances on a handwriting and a grip-strength task. Performance differences between the two sides for each group were not related to overall preference assessments but were positively associated with hand preferences on corresponding questionnaire items. Distributions of the performance differences between the two sides for the right- and left-handers were well separated for handwriting but completely overlapped for grip strength. Significant correlations between the performances on each side were recorded on both tasks. The results stress the importance of using the same criteria in making preference vs. performance comparisons and highlight the need to recognize and control the influence of previous training or experience.     

Adapting chronometric tests for field studies of cognitive ability

This study was designed to examine the relationship of hand preference to performance on a thumb-positioning task. Two hypotheses were proposed on the basis of hemispheric lateralization. The first stated that all subjects would perform the task more accurately using the left thumb, regardless of hand preference. The second hypothesis tested was that left-handed subjects would perform more accurately than right-handed subjects. Each of 20 neurologically intact adults grouped by hand preference reproduced a criterion thumb position with both thumbs simultaneously. Absolute error, recorded for each movement and analyzed by mixed analysis of variance, did support the first hypothesis. However, the left-handed group did not perform statistically differently from right-handed subjects. Factors influencing the data are discussed and recommendations for further studies are made.     

SALTNESS, SWEETNESS, AND PREFERENCE: A study of quantitative relations in individual subjects

Individual scales were constructed for saltness and preference in an experiment with eight subjects and for sweetness and preference in a second experiment with seven of these subjects. It was found (1) that power functions described the individual relations between sodium chloride and sucrose concentration, on the one hand, and the subjective variables of saltness and sweetness, on the other hand, (2) that the exponents exceeded 1 but varied greatly between subjects, and (3) that there was a great variation over individuals with regard to the form of the saltness/preference and the sweetness/preference relations.     

Posturally related variations in the hand preferences of the ruffed lemur (Varecia variegata variegata)

The hand preferences of 5 semi-free-ranging black-and-white ruffed lemurs were assessed by using three distinct testing procedures. Testing conditions varied in the extent to which they required animals to make a whole body postural adjustment prior to making a reach. Minimal bodily adjustment was necessary for free foraging, whereas discrete food presentations on land (DFP-land) and in a moat (DFP-moat) promoted a gross reorientation of the animal's entire body. In the DFP-moat condition 4 animals exhibited exclusive use of the left hand, and only 1 of 515 reaches was made with the right hand. Similarly, all 5 animals showed a pronounced left hand preference in the DFP-land condition. The free-foraging condition revealed a hand preference for only 1 of the 5 subjects, and that preference was weak in comparison with those measured in the other two test conditions. These findings indicate that whole body postural adjustments critically influenced the expression of hand preference and should be taken into consideration in future studies of primate hand preferences.     

Birth order and selected work-related personality variables

A possible link between birth order and various individual characteristics (e. g., intelligence, potential eminence, need for achievement, sociability) has been suggested by personality theorists such as Adler for over a century. The present study examines whether birth order is associated with selected personality variables that may be related to various work outcomes. 3 of 7 hypotheses were supported and the effect sizes for these were small. Firstborns scored significantly higher than later borns on measures of dominance, good impression, and achievement via conformity. No differences between firstborns and later borns were found in managerial potential, work orientation, achievement via independence, and sociability. The study's sample consisted of 835 public, government, and industrial accountants responding to a national US survey of accounting professionals. The nature of the sample may have been partially responsible for the results obtained. Its homogeneity may have caused any birth order effects to wash out. It can be argued that successful membership in the accountancy profession requires internalization of a set of prescribed rules and standards. It may be that accountants as a group are locked in to a behavioral framework. Any differentiation would result from spurious interpersonal differences, not from predictable birth-order related characteristics. A final interpretation is that birth order effects are nonexistent or statistical artifacts. Given the present data and particularistic sample, however, the authors have insufficient information from which to draw such a conclusion.     

Birth order and personality: Some empirical findings and a biobehavioral theory

The birth-order variables assessed included sex and number of siblings, assignment to 2- or 3-child family for separate sample analysis, family position as appropriate to given sample, spacing effect—was the birth of the second child spaced 4 years or more after the first child—dominance relationships in the family, sex of nearest siblings, sex of other siblings as appropriate, whether raised by both original parents, whether raised by foster parent(s). The personality variables assessed included sociability, anxiety, dominance, superego, phlegmatic temperament, involvement, self-regard and other scales measured by the new Howarth Personality Questionnaire. Samples of 50 for the 2-child families and 58 for the 3-child families were analyzed and it was found that: (a) in the 2-child families eldest children were less cooperative than younger, that those reporting dominance by another sibling were higher in state anxiety and this also applied to the younger of the pair, that those spaced in birth order were less afraid of being socially unacceptable, that those raised by both parents were higher in involvement; (b) in the 3-child families those dominated by a sibling, regardless of position, had more hypochondriac symptoms (a sign of anxiety), and that those spaced in birth order showed less cooperativeness. A biobehavioral theory to encompass biological-hormonal spacing effects and dominance as well as other intrafamilial behaviors was proposed.     

Cerebral Organization of Motor Programming and Verbal Processing as a Function of Degree of Hand Preference and Familial Sinistrality

Seventy-six right- and left-handed subjects responded to monaurally presented verbal stimuli (CVs) using their right and left hands on separate occasions. Both degree of hand preference and familial sinistrality (FS) were taken into account. It was found that, contrary to expectation, the manual response interfered with the verbal perception task, but only in the consistent strong handers. The pattern of interference suggests that those with a consistent hand preference (right or left) have general motor programming in the left hemisphere. Those with an inconsistent strong hand preference probably have some degree of general motor programming in both hemispheres. No effect for FS was found for the lateralization of verbal processing or general motor programming.     

Cultural Difference in Hand Preference: Evidence from India and Japan

Indian (N = 400) and Japanese (N = 502) adult subjects were examined for their hand preference on a 7-point scale (1 = left always, 7 = right always) of the 32-item Waterloo Handedness Questionnaire (Steenhuis & Bryden, 1989). Factor analyses of the data yielded a two-factorial structure of hand preference: skilled and unskilled. Interaction of Culture × Hand preference indicated that Indians had more right-hand preference for unskilled activities whereas Japanese subjects had more right-hand preference for skilled activities. Further analyses revealed that the frequency of middle category responses was more common in Japanese subjects. Indian subjects were found to give more extreme responses for either right- or left-hand preference. Findings are discussed in the light of cultural training given to the individuals in these two societies, which are essentially collective in nature.     

Is there an effect of handedness on the position of the egocentric reference?

Forty normal subjects (20 left-handed; 20 right-handed) were submitted to a proprioceptive straight ahead pointing task while blindfolded to study the effect of the hand used and of the hand preference on the estimation of the subjective sagittal middle. Results showed that left-handed and right-handed subjects both deviated to the left of the objective sagittal middle and presented an effect of the hand used and of the starting point affecting their straight ahead performance in a similar way. In all subjects whatever their manual preference, using the left hand and starting 30 degrees to left induced a tendency to deviate to the same side, whereas using the right hand and starting 30 degrees to right induced a tendency to place the subjective middle to the right of the objective middle. These results are interpreted with regard to the hypothesis of a cortical control of the position of the egocentric reference.     

Individual differences in right and left reaction time.

Experiments designed to check the absence of effects for hands and handedness in simple and two-choice reaction time found unexpected individual differences related to stimulus laterality. The majority of subjects responded faster to the stimulus on the left and a substantial minority responded faster to the stimulus on the right in any choice pair. The right index finger was slower than the left index or the middle fingers. Choices tended to be faster between fingers on different hands than on the same hand and same-hand choices were faster with the left hand than the right hand. There were no effects attributable to hand preference or sex.     

Visual line bisection in sinistrals and dextrals as a function of hemispace, hand, and scan direction

Visual line bisection was investigated in 26 sinistral and 24 dextral subjects as a function of hemispace, hand and scan direction. An ANOVA revealed significant main effects for hand preference, due to the mean bisection errors of dextral subjects being significantly leftward of those of sinistral subjects; for hand, due to the bisection errors of the left hand being significantly to the left of the right hand; and for scan, due to the bisection errors following a left scan being significantly to the left of a right scan. One significant interaction was found, that between hand and direction of scan, due to a significant difference between left and right hands following a scan from the left but not following a scan from the right. For dextral subjects the leftward bisection errors of the left and right hands following a scan from the left, but not for a scan from the right, differed significantly from the midpoint. For sinistral subjects the leftward bisection errors following a scan from the left and rightward bisection errors following a scan from the right differed significantly from the midpoint for the left hand but not for the right hand. No significant main effect or interactions for hemispace were found. This confirms that both sinistral and dextral subjects display pseudoneglect when using their preferred hand and scanning from the left. However, sinistrals, but not dextrals, will display reversed pseudoneglect when using their preferred hand and adopting a scan direction from the right. These results are discussed in terms of the interaction between three factors, whose influence can jointly and severally produce misbisections, hemispheric specialisation for visuospatial function, hemispheric activation for a manual response, and the allocation of visual attention.     

Development of hand preferences in marmosets (Callithrix jacchus) and effects of aging

The authors investigated the development of unimanual hand use and hand preferences during feeding in 15 marmosets (Callithrix jacchus), ages birth to 51-70 months. Bimanual hand use was common at 1-2 months, but by 5-8 months unimanual holding had developed and so had significant hand preferences. Half of the marmosets preferred to pick up and take food to the mouth with the left hand, and half preferred the right hand. Individuals maintained the same hand preference at all ages examined. Significant relationships were also found between the postures adopted during feeding and the direction of hand preferences displayed by juvenile marmosets. There was a positive correlation between increased suspension and increased left-hand preference, and a negative correlation between increased feeding in a tripedal posture and increased left-hand preference. These results are discussed in terms of motor development and hemispheric specialization.     

利手与内隐左右空间情感效价关系的眼动研究

为探明手动作流畅性和情感材料呈现空间在不同利手者左右空间情感偏好中的关系,本研究将情绪Stroop范式和眼动测量相结合,通过反应速度和眼动数据将动作流畅性和空间情感注意偏向相分离,并考察其交互作用。结果发现右利手个体的反应速度存在优势手效应,不同利手者在使用左手时表现出对优势手同侧空间的内隐情感偏好,表明右利手个体的反应速度存在优势手流畅性的主导作用,手动作流畅性和内隐空间情感偏好的作用可以分离。     

Human gestures trigger different attentional shifts in chimpanzees (Pan troglodytes) and humans (Homo sapiens)

Brain lateralization has been the matter of extensive research over the last centuries, but it remains an unsolved issue. While hand preferences have been extensively studied, very few studies have investigated laterality of eye use in non-human primates. We examined eye preference in 14 Campbell’s monkeys (Cercopithecus c. campbelli). We assessed eye preference to look at a seed placed inside a tube using monocular vision. Eye use was recorded for 100 independent and non-rewarded trials per individual. All of the 14 monkeys showed very strong preferences in the choice of the eye used to look inside the tube (mean preference: 97.6%). Eight subjects preferred the right eye and six subjects preferred the left eye. The results are discussed in light of previous data on eye preference in primates, and compared to data on hand preference from these subjects. Our findings would support the hypothesis for an early emergence of lateralization for perceptual processes compared to manual motor functions.     

Comparative and familial analysis of handedness in great apes

Historically, population-level handedness has been considered a hallmark of human evolution. Whether nonhuman primates exhibit population-level handedness remains a topic of considerable debate. This paper summarizes published data on handedness in great apes. Comparative analysis indicated that chimpanzees and bonobos show population-level right handedness, whereas gorillas and orangutans do not. All ape species showed evidence of population-level handedness when considering specific tasks. Familial analyses in chimpanzees indicated that offspring and maternal (but not paternal) handedness was significantly positively correlated, but this finding was contingent upon the classification criteria used to evaluate hand preference. Overall, the proportion of right handedness is lower in great apes compared with humans, and various methodological and theoretical explanations for this discrepancy are discussed.