The Effect of Birth Order on Intelligence
Sponsored by Missouri Western State University Sponsored by a grant from the National Science Foundation DUE-97-51113
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The proper APA Style reference for this manuscript is:
WALTON, J. L. (2001). The Effect of Birth Order on Intelligence. National Undergraduate Research Clearinghouse, 4. Available online at Retrieved April 25, 2017 .

The Effect of Birth Order on Intelligence

Sponsored by: MUKUL BHALLA (
The purpose of the present research was to determine whether familial birth order has an effect on intelligence. Forty-nine students (26 freshmen, 12 sophomores, 6 juniors, and 5 seniors) attending Loyola University in New Orleans were administered the Wonderlic Personnel Test (Wonderlic, 1983). Of these forty-nine participants 10 were male and 39 were female. Each participant indicated his/her age, sex, college level, ordinal birth position (firstborn, middle, lastborn, and whether adopted or fostered. Those participants that indicated they were adopted or fostered were omitted from this study. The middle born scored slightly higher than the firstborn, but not enough to be statistically significant. The middle born did however, score statistically higher than the lastborn, while the firstborn also scored statistically higher than the lastborn. Findings indicated that birth order was indeed related to individual intelligence.

There has been a lot of research related to the question of whether birth order does, in fact, have an effect on intelligence. Some individuals believe that older children in a family are more intelligent than the middle or younger siblings because the parents tend to be more overprotective with them and expose them to more "adult thinking." Others believe that parents are stricter with their firstborn and then become more lenient with the younger siblings. Perhaps this treatment would influence the intelligence levels of all the children. Finally, there are others who believe that the youngest child is favored by the parents, which would greatly enhance his/her intelligence due to the amount of time parents spend with them. Some studies have indeed found the younger siblings to be more intelligent than the older siblings (Steckel, 1930) while other studies have found just the opposite to be true (Zajonc and Markus, 1975 as cited in Cicirelli, 1995). Yet others, such as Pillai and Ayishabi (1984), have found no relationship between birth order and intelligence. In a study conducted by Pillai and Ayishabi (1984), 532 college students were placed into five groups according to their birth order and sex. Birth orders 1, 2, 3, and 4 were separate groups and 5 and above were placed together in one group. They were given the Kerala University Group Test of Intelligence for Adults to see if there would be a difference in the mean scores of intelligence between the different groups. The conclusion of this study found that birth order had no influence on intelligence. The majority of studies that have been performed do, however, find some relationship between birth order and intelligence. Some of these studies support the premise that the lastborn child is more intelligent than the firstborn. For example, in the study performed by Steckel (1930), questionnaires were distributed to 2,712 families that contained a total of 6,790 children. Due to such a large sample, the sample obtained was completely dependent on the willingness of the parents to answer the questionnaires. The study concluded that the average intelligence of laterborn children was higher than that of the earlier-born children and that intelligence increases with ordinal number in a family. On the other hand, there are studies that have found the opposite to be true. Burton (1967), in her study, found that for two-, three-, four-, and five-children families the intelligence of the oldest siblings appeared to be slightly higher than that of the younger siblings. However, the mean difference of the standardized intelligence scores between the older siblings and the younger siblings showed only a small difference. This study concluded that the difference in intelligence did not appear to be significantly large enough to account for the large differences in achievement due to birth order. Besides birth order, many outside factors could influence how an individual develops intellectually. For instance, where an individual attended school could affect his/her intellectual performance. Another variable could be a particualar area in which one has lived. Living in a certain area could either have a negative or positive affect on the individual. Some people may have been more advantaged than others and had better access to a better education. Any of these variables could have a large impact on an individual, and may be tied to the effects of birth order on intelligence. Based on the results mentioned above, some researchers have questioned whether there is any relationship at all between birth order and intelligence. In a review by Manaster (1993), he questioned whether birth order was defined the same way in all studies. If not, contradictory results could possibly occur. He also mentioned that many could mistake ordinal position for birth order position. Ordinal position was referred to in this study as the specific rank or order in a numerable series (such as first, second, or third). Birth order was defined as a category or type of person whose distinctive character can be made known or demonstrated (such as only, oldest, second, middle, and youngest). Another study by Rodgers, Cleveland, Oord, and Rowe (2000), supported this same idea, as cited by APA Public Communications ( In this study these researchers found that many other factors, such as family size, parental IQ and genetic heritage, could also significantly influence intelligence in children, rather than just birth order alone. Although intelligence can be influenced by factors such as family life, geographical location, social class and level of education, the main focus of this study was to determine whether or birth order had a major effect on how much one learned and how well he/she developed intellectually. The current study examined whether birth order had a significant influence on an individual`s intellectual achievement. It was hypothesized that birth order would have an effect on the intelligence level of an individual. The intelligence test results taken from these college students opened a new door of opportunity and further advanced past research and helped improve ideas and attitudes of those who were interested in birth order and the effect it had on individuals.

ParticipantsForty-nine college students (10 males and 39 females)over the age of 18 attending Loyola University in New Orleans participated in the study. They were obtained through convenience sampling. Some participants received extra credit from their psychology professors for their participation in the study. MaterialsAll participants were asked to read over and sign an informed consent form, which briefly described the purpose of the study. The participants were then administered a sheet of questions which asked their: sex, age, college level (freshman, sophomore, junior, or senior), and if they were adopted or from a foster family. They were also asked whether they were the firstborn, middle (i.e. not the first or last-born), or last (lastborn) sibling in their family. Single children were placed in the group along with the firstborn children. An intelligence test was used, called the Wonderlic Personnel Test (Wonderlic, 1983) in order to measure individual intelligence. It was in a multiple choice/fill in the blank questionnaire form and consisted of a total of 50 questions (e.g. REAP is the opposite of 1. obtain, 2. cheer, 3. continue, 4. exist, 5. sow). The scores could range anywhere from a 0-50 with a standard deviation of approximately 7 points. This test had a time limit of 12 minutes.Design and Procedure This study focused on how birth order affects intelligence. The study was a single-variable, between-subject, quasi experiment. The dependent variable was the intelligence level of each participant as measured by the Wonderlic Personnel Test. The independent variable was birth order. Birth order was defined as the oldest, middle, or youngest sibling in a family. The participants in this study were children from direct biological descent. Those who were adopted or part of stepfamilies were omitted from this study. Intelligence level was obtained from the score of each participant on the Wonderlic Personnel Test. Each group was seated comfortably in a classroom located on Loyola`s campus. Each individual read and signed an informed consent form. Once the informed consent forms were completed, the researchers collected them from the participants and then administered the demographic questionnaire and the Wonderlic Personnel Test. The participants were instructed to answer all 50 questions by circling the best answer or filling in the blank for each question. The participants were allowed 12 minutes to complete this test. They were then asked to put their pencils down. The researchers then collected the materials and debriefed the participants. The researchers explained to each group of participants that their participation would help to further explore the effect of birth order on intelligence. They were also assured that the results of the intelligence test would be kept confidential to avoid embarrassment in the event of low scores.

The sample used in this study contained a total of 10 males (1 lastborn, 1 middle born, and 8 firstborn) and 39 females (9 lastborn, 10 middle born, and 20 firstborn). A one-way analysis of variance was used to compare the mean scores of the firstborn, middle born, and lastborn siblings on the Wonderlic Personnel Test (1983). This was found to be statistically significant. F (2, 46)=4.370. p<.018. LSD posthoc tests indicated that the mean for the middle born (27.0, SD=4.20) was not significantly greater that the mean for the firstborn (24.75, SD=4.54). However, the mean for the youngest (21.3, SD= 4.42) was significantly lower than the mean for either the firstborn or middle born.

The purpose of this study was to determine whether birth order has any affect on intelligence and, if such a relationship exists, to determine, if possible, which ordinal position is superior. In the sample of 49 college students that participated in this study, differences in intelligence did appear significant between the middle born and lastborn versus the firstborn. These findings are not consistent with all the research in this area. Steckel (1930) found a general relationship between intelligence and birth order of Caucasian public-school children and, that on the average, later-born children score higher on intelligence tests than earlier-born children. She further found that intelligence increases at a consistent rate up to and including the eighth-born child. Burton (1967), on the other hand, found the opposite to be true. She found the intelligence level of the firstborn to be higher than the last born, with the middle born to be somewhere in between. However, Burton felt other factors, such as economic and social variables, should be considered. A June 2000 study by Rodgers, Cleveland, Oord, and Rowe interpreted data from the National Longitudinal Survey of Youth and found no direct causal link between family size, birth order, and children`s intelligence. Although past research, mentioned above, has found a higher level of intelligence for firstborn or for lastborn siblings, this study has found a higher significance in the intelligence level of the middle born sibling. This opens a new door of opportunity for future research on birth order and intelligence. Instead of following the "trend" of either the firstborn or lastborn sibling having the higher intelligence level, more emphasis should be put on the middle born sibling. Another factor that may be taken into consideration for future research would be to study siblings within a family rather than between many different families. This would help eliminate some cultural or environmental differences between the various groups of siblings. Studies have analyzed people such as Yeltsin, Clinton, Castro, and Shakespeare and their birth orders to determine if there are similar characteristics they share. Many differences do exist between various siblings in a family. While individual intelligence and development may be affected by gender, family size, and age spacing as well as by factors such as parental expectations, sibling order should be considered as the basis upon which other causal factors may operate. Because of the obvious inconsistency in birth order research results, more detailed and more frequent research should be conducted. Additionally, researchers should be careful not to make broad generalizations and predictions based on data that is relatively limited and inconclusive. Certainly, this area of study would not be used to predict that an individual who occupies a particular birth position (firstborn, middle born, lastborn) is more or less intelligent, or more or less likely to succeed. Birth order can also be another factor to consider when analyzing human personality. One must realize that birth order may be only one influence among many, as is true with intelligence. The way parents treat their children as well as the atmosphere in which they live affect human personality also. Birth order information does not give the total psychological picture for any child. There is no system of personality development that is capable of that. Birth order statistics and characteristics provide indicators that combine with psychological, mental, and emotional factors to give the bigger picture. Birth order principles do not automatically solve problems or change personalities overnight. However, knowledge of birth order differences would be useful in school settings to explain why certain children perform as they do in the classroom. Additionally, birth order could be used in a therapist/patient setting where difficult problems related to family adjustment, sibling rivalry, and parental relationships are a factor. Theories explaining birth order and its relationship to intelligence and personality have inherent strengths and weaknesses. Only through more thorough research can accurate conclusions be drawn.

Burton, D. (1967) . Birth order and Intelligence. The Journal of Social Psychology, 76, 199-206. Circirelli, V. G. (1995) . Sibling Relationships across the Life Span. New York, N. Y: Plenum Plus Publishing Corporation. Manaster, G. J. (1993) . Birth Order: An Overview. The Journal of Individual Psychology, 33, 3-8. Pillai, K. S., & Ayishabi, T. C. (1984) . Influence of Birth Order on Intelligence of College Students. Psychological Studies, 29, 172-174. Rodgers, J. L., Cleveland, H., Oord, E. V. D., & Rowe, D. (2000) . Resolving the Debate Over Birth Order, Family Size, and Intelligence. [On-line] . American Psychologist, 55. Abstract from: APA News Release:, M. L. (1930) . Intelligence and Birth Order in Family. The Journal of Social Psychology, 1, 329-344.Wonderlic E. F. (1983) . Wonderlic Personnel Test. Northfield, Illinois: Wonderlic and Associates Inc.

Submitted 5/4/01 10:51:56 PM
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