I read the journal article regarding the test of a research that had been conducted in 1975 by R.B. Zajonc and G.B. Markus. The author’s critiquing Zajonc and Markus’ experiment were Harold Grotevant, Sandra Scarr, and Richard Weinberg. Their article was titled ‘Children: A Test of the Zajonc and Markus Model’. Unfortunately, there were not any recent articles that I could find on this specific subject matter, so it was necessary to report from a 1977 research.
Zajonc and Markus conducted their research to determine if the first-born child in a family were born with higher IQ characteristics, and if so, why? It was found in their study that first-born children tend to be more verbally articulate, less impulsive, more active, higher performers in school, more apt to attend college and are more inclined towards achievement. It was also found that earlier-born children tend to score higher on intelligence and aptitude tests than those born later into the family.
Zajonc and Markus began their research based on the premise that first born and subsequent-born children are born into two different environments. For instance, the first-born enters a world of just two adults (normally). Whereas the second-borns’ environment is drastically different in that it consists of two adults and one child. So now the attention is not concentrated on one child, but two, or possibly more.
Zajonc and Markus applied their theory to a set of data information that had been gathered and published by researchers Belmont and Marolla. This is considered an unusual method, as they never came into contact, or had observation, with any of the Belmont and Marolla test subjects. Belmont and Marolla gave 350,000 Dutch males a Raven test, which is similar to an I.Q. test. They found a strong relationship between the birth order of the men and their scores on the Raven test. It seems that the scores decreased as a family size increased and also declined with birth order.
Using the information provided by Belmont and Marolla, Zajonc and Markus proposed the theory that as a family size increases, the average intellectual status of the entire family decreased. Zajonc and Markus applied the data from the Belmont and Marolla study into a table that provided information about the number of children and the average intellectual score for the children. They noticed that in larger families there is somewhat of a leveling off or even an increase in scores for the extreme later-born children. Zajonc and Markus also found that twins and triplets performed lower on tests of intelligence than non-twin siblings. Zajonc and Markus did not try to claim that the only determinates of intellectual ability were family size and birth order. There were other factors such as genetic heritage and prenatal care that additionally contribute to intellectual development.
Grotevant, Scarr, and Weinberg finalized their test of the Zajonc and Markus model by
concluding: “Family constellations are not powerful predictors of IQ differences when sources of variance within the family are taken into account. Just as the birth rate cannot predict whether Joneses or Smiths will reproduce, the confluence model cannot predict whether first- or fourth-borns in smaller or larger families will be brighter, when both genetic and environmental variables within and between families contribute to their IQ differences.”
Grotevant, Scarr, and Weinberg’s test involved adoptive and biological family units, used all genders, races and ages (instead of only Dutch eighteen-year-old males) and gave all the family age ranges age-appropriate IQ tests. Additionally, birth order, child spacing and family size were determined by interviewing both the adoptive parents and research of the adoptive records. It appears that the Zajonc and Markus model was considered poorly fit to the data on IQ individual differences. In addition to being extremely thorough, Grotevant, Scarr, and Weinberg indicated that the individual sources of variation in the Zajonc and Markus model were “ignored or minimized”.

While I would like to be able to agree with the Zajonc and Markus model, especially since I am a first-born and my younger siblings are twins, the Grotevant, Scarr, and Weinberg test of this model was considerably more thorough. Though in the end neither took into consideration the variable of work quality and motivation. The older sibling might be more intelligent, but that does not mean they are a harder worker, or more motivated, than the last-born sibling.

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