d.2 Birth Spacing as Preventive Medicine for Sibling Rivalry (R. Proposal)

This Research topic submitted by Katie Johnson (johnso37@muohio) on 2/27/98.


Sibling rivalry is a phenomenon that fascinates our culture and is the subject of fiction and Satellite Movies as well as how-to parenting magazines. Our Evolutionary past sheds some light on the world of sibling relationships- why some siblings exhibit altruistic behavior and some exhibit a ongoing competitive attitude. Parents want to prevent their offspring from developing antagonistic especially in our culture today, where small successful families have replaced large ones.
From examining our Evolutionary tendencies and psychological studies, I have gathered that one strategy in preventing sibling rivalry is large birth spacing. A major factor, however, in sibling relationships (in an Evolutionary perspective) is also the reSources available to the siblings, and logically the more reSources there are, the more children can be thrown together with minimal conflict. On the other hand, smaller reSources would accommodate few successful, peaceful offspring, and large birth spacing would facilitate this situation.
I am conducting a Study to test the resource size, family size and birth spacing to understand the conditions under which large birth spacing would be beneficial. I assert that siblings with large birth spacing and low reSources will dramatically less sibling rivalry than those with low reSources and small spacing. On the other hand, siblings with large parental reSources and small spacing will still be more rivalrous than those with larger spacing, but not so drastically as those families with small reSources. If this turns out not to be the case, our Evolutionary need for parental reSources is overshadowed by social and cultural influences on sibling relationships

Theoretical Background

The Research behind this claim begins with Hamilton’s law of kin selection that is still being tested and studied by scientists. His premise is that your sibling has half of your genes, so you are more likely to help him/her than someone unrelated to you, but you will help yourself first. This can be expressed in the following equation:
c < br
c= the cost of altruistic behavior
b= the benefit to kin of altruistic behavior
r= the percent of relatedness (for human siblings r= .5) (Wright 170)
One recent study on this theory revealed how in a certain population of salamanders in which one will occasionally break out and start eating his neighbors, this cannibal morph appeared less often and later in a tank made up relatives. Also, in tanks of mixed families, the morph would eat its non-kin before its kin. This illustrates how inclusive fitness works and what makes sibling relationships so complex (Gonnick 124).
The question remains: How do we use this basic principle for kin relationships to make environments that lower the cost of altruism, and therefore maximize altruistic feelings. From examining ethnographic and psychological studies of sibling relationships it seems that when siblings are spaced far apart, the older sibling takes on a socializing/caregiving role that curbs rivalry. On the other hand, closer spaced siblings form more intense bonds and more intense conflicts.
Many of these studies are cross-cultural examination of sibling behavior. Patricia Zukow, for example, studied low-income Mexican families, and came to the conclusion that, “Siblings act as socializing agents throughout the world” (Zukow 80). Another study examined the Kwuarua culture in the south Pacific islands. This study revealed how important older sibling were in giving child care, and there sibling rivalry was practically non-existent (Gregeo 75).
Large birth spacing makes a big difference in how children react to these roles of teacher and novice. One psychological study rated the playing of sibling pairs with varying age differences by its effectiveness in socialization. Topics like “linguistic skills” and “artistic development” were used to distinguish the types of play, and not surprisingly, the pairs with larger age differences engaged in more effective play (Bond, Gibbs, and Teti 166-7).
Frank Sulloway supports this assertion that large birth spacing decreases sibling rivalry in his book, Born to Rebel. While this book (quoted by Wright), focuses on how the younger sibling can project his role in the family (fighting into the pre-made establishment) onto his/her future career and ideology, he also notes how birth spacing affects this phenomenon. He describes how older sibling of newborn birds will, with the help of the mother, kick the rookie out of the nest (Sulloway 133).
His main point deals with how younger siblings find individual “niches” to gain the attention of parents without going for the same resource area as an older sibling. Sulloway shows how this happens more successfully in families of large-spaced children. While younger siblings generally have more revolutionary mindsets than their older siblings, this trend was more pronounced in siblings of differing ages. Large birth spacing can pave the way for siblings to find their niches more easily (Sulloway 134).
Popular parenting magazines offer the same kind of advice for stopping sibling rivalry such as not to compare siblings and not to take sides in a fight. Perhaps a more preventive form help would be useful considering our tendency to compete with siblings for resources while protecting them as carriers of half our genes. Large birth spacing can be worth the reproductive costs for parents by putting their offspring in a position to help each other with little cost, and in a position to find successful individual niches.


As I mentioned earlier, I will conduct a survey to test for the resource size, family size and birth spacing. The results of this survey constitute the evidence to prove or disprove my hypothesis I will survey about 40-50 individuals about their family situations. The first part of my survey will deal with resources which I will measure by asking about the number of parents in the household and the household income while growing up.
I will then ask the participants how many siblings they have, and the differences in the ages. I also want to know if either the participant or his/her sibling are handicapped or were requiring special medical attention. Lastly, I want to asses the nature of the participant’s relationship with his/her sibling. In the case of multiple siblings, I will ask the participant to answer questions in terms of the sibling closest in age. This is the most difficult part of the study, since individuals’ relationships vary so greatly. The survey will contain questions concerning the participants memories of playing with their siblings, and how the relationship has developed into adulthood.


This survey should contain about 20 questions, and I hope to show how resources and sibling numbers and spacing correlate. If my predictions hold, then the participants with one parent at home, and small income should show more rivalrous attitudes towards their siblings and even more rivalrous attitude if the family is large and the birth spacing small. If their is no pattern in where sibling rivalry rears its ugly head, then it is mainly an issue of personality, family, or cultural influence.
I will show my results by putting the factors together on in Stat-view much in the fashion of the survey at the beginning of the semester.


Gonick, Larry. “Fine Young Cannibals.” Discover. October, 1993. pp. 124-5.

Kent, Debra. “How Even Good Parents Fuel Sibling Rivalry.” Good Housekeeping.
March, 1997. pp.80-3.

Leder, Jane Mersky. “Adult Sibling Rivalry.” Psychology Today. January, 1993.
pp. 56-(8 pages).

Samalin, Nancy. “How to Cure Sibling Fights.” Parents. May, 1993. pp. 146-50.

Sulloway, Frank J. Born To Rebel. Pantheon Books: NewYork, 1996.

Teti, Douglas; Gibbs, Elizabeth; and Bond, Lynne. “Sibling Interaction, Birth Spacing,
and Intellectual/Linguistic Development.” from Sibling Interaction Across Cultures.
edited by Patricia Zukow. Springer-Verlag: New York, 1989.

Watson-Gregeo, Karen Ann and Gregeo, David W. “The Role of Sibling Interaction in
Child Socialization.” from Sibling Interaction Across Cultures. edited by Patricia Zukow. Springer-Verlag: New York, 1989.

Wright Robert. The Moral Animal. Vintage Books: New York, 1994.

Zukow, Patricia G. “Siblings as Effective Socializing Agents: Evidence From Mexico.”
from Sibling Interaction Across Cultures. edited by Patricia Zukow. Springer- Verlag: New York, 1989.

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