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Brittany Hambleton
Brittany Hambleton
January 20, 2024 ·  2 min read

Science Says Your Younger Sister May Be Making You Fat

Having a younger sister can be a wonderful thing. She can be like a built-in best friend, someone to share a wardrobe with, and can often act as an easy scapegoat to blame when you get in trouble.

If you have a younger sister, science says you may have something else you can blame her for: she might be making you fat.

Your Younger Sister Might be Making You Fat

There have been a number of studies that have shown differences in body type between first-born and later-born men, mainly that the older sibling tended to have a higher BMI than his younger brother [1]. 

In 2015, a group of scientists from Sweden set out to determine if the same is true for women by conducting a study. They started by collecting clinic records from the Swedish National Birth Register from 1991 to 2009 and assessed 13 406 pairs of sisters, measured their weight and height, and collected data on their current health, lifestyle, and family history [2].

The researchers found that firstborns were actually lighter than their younger sisters at birth, but were 29 percent more likely to be overweight and 40 percent more likely to be obese as they got older.

The number of children in a family was not associated with an increase in body mass index (BMI), however having more siblings was associated with shorter height. This is possibly due to what researchers call the “resource dilution hypothesis”, which suggests that the more children in a family, the less resources there are for any one child [3].

Read: Having a Sister Is Beneficial To Your Mental Health, Studies Show

How Can this Be?

Science says that there is growing evidence that firstborns may be at a higher risk for health problems like diabetes and high blood pressure in adulthood, but the cause of this still remains somewhat of a mystery.

One possible reason is that a mother is more likely to overfeed her first baby, and that overfeeding continues. Another is that the firstborn competes with their younger sibling for food, or that more money gets spent on the firstborn child [4].

Whatever the reason, it is important to note that all of this data is observational, and more research needs to be done to determine exactly why older siblings seem to be more at risk for being overweight.

Read: The Pros and Cons of Being a Second-Born or Middle Child

Family Size and Obesity

One interesting observation made by the researchers is the possible link between the reduction in family size and the overall increase in obesity around the world.

“The steady reduction in family size may be a contributing factor to the observed increase in adult BMI worldwide, not only among men, but also among women,” they concluded [2].

So if you’re an older sister (or brother), it may be tempting to blame your younger sibling on your weight gain, but in reality it is not so much because they are younger that you are more prone to being overweight, but rather because you were born before them.

Keep Reading: Evidence Your Older Sibling Is Probably Smarter Than You