Final: Birth Spacing As preventive medicine for sibling rivalry

This research topic submitted by Katie Johnson ( ) on 5/6/98 .


Sibling rivalry is not a new or surprising event in the family. Cain and Abel, Romulus and Remus, and Cinderella and her stepsisters all suffered from this pattern in sibling relationships. In my study, I closely examined sibling relationships, and more specifically, the reasons behind and results of sibling rivalry in these relationships.
From my research in evolutionary psychology, sibling rivalry is a natural part of having more than one offspring, for offspring tend to compete over the parents¹ resources to secure their individual survival. Hamilton¹s law complicates this a bit by showing how siblings tend to be somewhat altruistic to each other as well as being mainly self-serving because of the amount of genes shared by siblings. The bottom line is that parents want as many of their genes to continue in all of their offspring, while siblings tend to want to fight somewhat over the parental resources. Conflict all around.
In the interest of the parents, I tried to find a way to exploit the natural altruism between siblings and minimize the rivalry that can be detrimental to maximal gene continuance (the Cain and Abel situation). I researched the conditions under which sibling rivalry did and did not occur in this and other societies, and I combined these conditions with evolutionary strategies for controlling rivalry. As a result, I hypothesized that large birth spacing, in general, would lessen sibling rivalry, and through survey techniques I tested for this condition, and I tested for conditions of parental resources under the hypothesis that high resources would mean less rivalry.
Unfortunately, while some of my data initially agreed with my first hypothesis, the p-values showed that there was little correlation between the variables I chose to represent the parts of my hypotheses. The data completely refuted my second hypothesis on resources, and the factors of competition and the factors of resources showed little correlation. I had to conclude that either my testing techniques are poor, or sibling rivalry cannot be controlled by this evolutionary strategy, but by dealing with individual personalities and family situations.
In this study I set out to test two hypotheses concerning plans of action in combating sibling rivalry. The central hypothesis was that large birth spacing would curb the tendencies for siblings to form rivalries relationships. This hypothesis came from evolutionary psychological observations on sibling rivalry and further research on psychological and ethnographic studies in sibling relationships. Also as a result of evolutionary psychological principles, I hypothesized that sibling rivalry would occur less frequently in high resource situations.
The research behind these claims begins with Hamilton¹s 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 keep their genes going. When altruistic behavior occurs 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, more closely spaced siblings form more intense bonds and more intense conflicts.
Many of these studies are cross-cultural examinations of sibling behavior. Patricia Zukow, for example, studied low-income Mexican families, and came to the conclusion that, ³siblings act as socializing agents throughout the world² (Zukow 80). Another
study examined the Kwuarua culture in the south Pacific islands. This study revealed how important older sibling are 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 ³linguistic skills² and ³artistic development² 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). Although larger space between siblings ages seems to cause less rivalry, there is less intimacy as in siblings of close ages (Leder 58).
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 siblings can project their role in the family (fighting into the pre-made
establishment) onto their future career and ideology, he also notes how birth spacing affects this phenomenon. He describes how older siblings 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 ³niches² 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). It stands to reason, then as well, that the more resources in a family structure, the less chances there are for conflict between siblings over resources. A sibling will be able to get sufficient resources without needing the other siblings¹.
Popular parenting magazines offer advice for stopping sibling rivalry such as not to compare siblings and not to take sides in a fight that coincide with the Sulloway¹s assertion on niches. Perhaps a more preventive form help would be useful on combating rivalry considering our tendency to compete with siblings for resources while protecting them as carriers of half our genes.
Large birth spacing however, has serious consequences for successful gene continuance. When children are spaced far apart, considerably less gene carrying offspring can be produced. The severity of this consequence can be seen in the table below, which show the offspring numbers in a sequence in which each member has two offspring as opposed to a sequence in which each has three:

Numbers of Total Offspring
two per offspring three offspring

1st generation 1 1
2nd 2 3
3rd 4 9
4th 8 27
5th 16 81
6th 32 243

As you can see, after just six generations there is a substantial loss in gene volume. Large birth spacing needs to be able to make up for these costs by having a big effect in making sibling relationships helpful instead of harmful and by putting siblings in a position to create successful niches for themselves as Sulloway describes.
If my empirical study fails to show that the factor of birth spacing creates a less competitive relationship between siblings, than my alternative hypothesis would be that preventing sibling rivalry is more a matter of each parent working with the individual personalities of his/her children, which is the basis for the actions suggested by parenting magazines, although they show some of the general advice falls in the trends of evolutionary psychology. Basically, if my hypotheses are refuted, it shows that individual traits are more important than the biological trends in solving the problem of sibling rivalry.

To carry out this study, I prepared a survey that asked the following questions:
1)Please give the following information concerning all of the siblings in your family as you were growing up including yourself:

Birthdate or age Gender Marital Status Current job
(banker, student etc.)

2) Please estimate the yearly income in your household as you were growing up:

less that $25,000 $25,000-$45,000 $45,000-$65,000
$65,000-$85,000 $85,000-$100,000 More than $100,000

3)How many parents were in the home while you were growing up?

On a scale of one to ten:

4)How intimate were you with the sibling closest in age with you?

With the sibling farthest in age with you?

5) How competitive were you with the sibling closest in age with you?

With the sibling farthest in age with you?

I gave this survey to thirty-six students at Miami University. Of course I couldn¹t give it to people with no siblings, and I added the stipulation verbally to only answer the closest/farthest questions once if the respondent had only one sibling. All of the respondents were between the ages of seventeen and twenty-six. Other problems in the survey were family situations in which there were different numbers of parents at different times, and few accurately knew their household income.
With the first set of questions I could see how many siblings were in the household, their birth spacing, and some other information that I collected in case I wanted to examine some other factors that might affect this study. The second question was a measure of resources; as was the third question. The remaining questions were primarily to measure the competative feeling between the respondent and his/her sibling(s). I added in the questions on intimacy to back up this since these should also be higher in closer sibling ages.
In compiling the data, I created the following categories to record each survey: number of siblings, age difference in closest siblings, age difference in farthest, intimacy rating for closest, intimacy rating for farthest, competition rating for closest, competition rating for farthest, average competition of both, income, and number of parents. If the respondent had only one sibling I simply recorded his/her answer in both the ³farthest² and ³closest² columns. I addressed the problem of changing numbers of parents in households by giving .5 or .75 value to a parent around some of the time. Also, on a separate data sheet, I combined the two ³age difference² columns, and, keeping coordinating answers in the same row, I added together the two ³competition² columns right next to it.
I began by graphing and analyzing these last two columns by graphing mean competition, splitting by age difference, and running an ANOVA test on these two variables. I then compared the mean intimacy ratings of closest and farthest siblings, and I compared the mean competition ratings for closest and farthest siblings. I then ran t-tests on both these pairs to get p-values.
These addressed my first and central hypothesis. I then worked with my second hypothesis concerning family resources and sibling rivalry by graphing the mean competition of all relationships and splitting by income. I subsequently split the mean competition by number of parents and by number of total siblings in the household. I ran ANOVA test on each of these comparisons to get p-values.

1) Levels of Competition in Sibling Pairs of Different Age Spacings
This graph refutes my hypothesis that competition should decrease as age difference increases. There is no pattern, and most of the averages hover around 6. The p-value (.3113) indicates that there is no significant correlation between competition and age difference.

2) Average Ratings From on to ten for INTIMACY and COMPETITION for Siblings CLOSEST in age and FARTHEST in Age

Initially, these comparisons seemed to support my hypothesis. The level of competition of the farther spaced siblings was slightly higher than the closer spaced ones, and the level of intimacy was drastically higher. The t-test results were again disappointing. The p-value for the competition comparison was .4789, indicating that there was no correlation between these factors. On the other hand, the intimacy p-value showed up as .0001, which shows that high intimacy did correlate with close sibling ages. Not surprising.

3) Average Sibling Competition in Different Yearly Income Brackets
This is the beginning of the testing for the effects of resource amounts on sibling rivalry. This graph did not support my hypothesis, but it did show a steady, bell curve shape. Competition was highest in the middle income bracket and lower in both the highest and the lowest brackets. Unfortunately the p-value was, once again too high. At .8055, there is very little likelihood of correlation between these variables.

4) Average sibling Competition with Different Numbers of Parents
This was another method to measure resources in comparison with competition levels between siblings. As in the last graph, the middle resource bracket was showed the highest level of competition. Once again, however, the p-value of .4497 indicated that there is little or no significant correlation between the number of parents and the competition between siblings.

5) The Average Competition in Different Sibling Group Sizes
This is the final attempt to connect the size of resources available to a pair of siblings to how competitive the siblings feel in relation to each other. The graph showed the opposite trend until hitting five siblings. The p-value of .4146 indicated again that this comparison was insignificant because there is no correlation between the variables.

Based on my empirical research, there is no reason to believe large birth spacing would prevent sibling rivalry, for both tests comparing birth spacing and competition between siblings showed little no significant correlation between these factors. My second hypothesis was also stoutly refuted by my empirical research. I examined three factors of household resources, but none them showed any correlation between competitive attitudes between siblings.
There are many places where this study was flawed, but the most significant was how the number of people surveyed compared with the ways a split the responses. For example, in Levels of Competition in Sibling Pairs of Different Age Spacing, there were 72 pieces of data that were split into fourteen bars. This kind of ratio allows indicates that any apparent patterns or correlation were based on chance.
My extensive interdisciplinary research illustrates a line of reasoning that led to my hypothesis, and it leaned strongly on the assertion that older siblings can take on a parenting role towards the younger sibling. This is a specific situation that is influenced by many factors. I ought to have asked on my survey if such a situation occurred in the respondent¹s household to see if where my reasoning was flawed in coming to this hypothesis.
In studying if resources affected sibling rivalry I was frustrated to find that none of my measuring techniques would correlate with competition, but again, my small sample could be to blame. In doing a study of this kind, however, there are so many factors involved in how many resources are available to an offspring such as his/her personality or the attentiveness of the parental figure. There is not a way to accurate isolate the factors one wants to pinpoint as socio-biological, which can explain the dirth of evidence Wright used in his book.
To conclude, I would like to draw on Chris Wolfe¹s problem with the study of sociobiology, that it is ³light on generating testable hypotheses.² I would certainly agree. To come close to accurately testing this hypothesis, I would have to sift through the cultural peculiarities of each person to understand how evolutionary principles have been translated through that person¹s psyche. From this perspective, problems like sibling rivalry should be addressed through a cultural as opposed to a socio-biological understanding.


Gonick, Larry. ³Fine Young Cannibals.² Discover. October, 1993. pp. 124-5.

Kent, Debra. ³How Even Good Parents Fuel Sibling Rivalry.² Good Housekeeping.
March, 1997. pp.80-3.

Leder, Jane Mersky. ³Adult Sibling Rivalry.² Psychology Today. January, 1993.
pp. 56-(8 pages).

Samalin, Nancy. ³How to Cure Sibling Fights.² Parents. May, 1993. pp. 146-50.

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

Teti, Douglas; Gibbs, Elizabeth; and Bond, Lynne. ³Sibling Interaction, Birth Spacing,
and Intellectual/Linguistic Development.² from Sibling Interaction Across Cultures.
edited by Patricia Zukow. Springer-Verlag: New York, 1989.

Watson-Gregeo, Karen Ann and Gregeo, David W. ³The Role of Sibling Interaction in
Child Socialization.² 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. ³Siblings as Effective Socializing Agents: Evidence From Mexico.²
from Sibling Interaction Across Cultures. edited by Patricia Zukow. Springer- Verlag: New York, 1989.

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