Are you an only child; the oldest of three; the youngest of five?
Your birth order can tell you more than just your place in the family, it can also influence many factors that determine your overall health and well-being.
For example, your birth order may be reflected in your personality and character traits, your height, and even your intelligence. It also gives clues as to which health problems you’ll be prone to.
Researchers have been fascinated by the effects of birth order for well over a century. A survey of English scientists carried out in 1874 found that many more of them were firstborns than not, suggesting they were more intelligent than their younger siblings.
Since then, many studies show this to be true. In fact, performances on intelligence tests decline slightly from firstborns to laterborns. This effect has been shown repeatedly and seems convincing.
Personality effects are not so clear, however.
Personality and height
Psychologists published a study in 2015 using data on over 20,000 adults from three countries. They found no effects of birth order on extraversion (traits such as talkative, social and outgoing), emotional stability, agreeableness, conscientiousness, or imagination. They concluded that “birth order does not have a lasting effect on broad personality traits outside of the intellectual domain.”
On the other hand, in a large dataset of Swedes, earlier born men were found to be “more emotionally stable, persistent, socially outgoing, willing to assume responsibility, and able to take initiative than laterborns.”
So, it looks like the jury is out on personality.
Birth order is also an important determinant of height. Birth orders two, three and four were shown to be an average of 0.4, 0.7 and 0.8 cm shorter respectively compared to the oldest child, among 652,518 Swedish men.
Height is indicative of more than just physical appearance. Height in and of itself is linked to multiple diseases.
Birth order, height and illness
Taller people have a higher risk of atrial fibrillation and varicose veins, and a lower risk of coronary heart disease, high blood pressure and high cholesterol. Birth order also influences many other physical conditions.
There are multiple studies looking at the relationship between birth order and health, but they mostly focus on health-related behaviors in children and teenagers. Results in adults are suspect because of their small study size.
Once very large health surveys linked to population registers became available, researchers were able to decipher patterns, one of which is that there’s no clear firstborn health advantage.
Firstborns are heavier
On the contrary, firstborns are more likely to be overweight and obese, and to have high blood pressure and high triglycerides (blood fats).
However, they are taller, have better mental health, and lower suicide rates.
Laterborns are more likely to have poorer self-reported physical and mental health.
Information on those without siblings is limited, but a just-published study used Swedish data from 1940 to 1975 involving people aged 17 to 50 or more. They reported that: “Only children showed lower height and fitness scores, were more likely to be overweight/obese in late adolescence, and experienced higher later-life mortality than those with one or two siblings.”
Are differences in birth order real or the result of other factors? Too much is unknown to answer that question with any clarity.
Why birth order matters
A higher economic status, education and income give people an advantage when it comes to health, but these factors are considered and adjusted for by researchers. Even after doing so, substantial variations in health still exist between siblings.
Information on biological factors is lacking but one possible mechanism is concerned with changes in the womb environment or in the maternal immune system that occur over successive births that may give firstborns an advantage.
Parents’ attitudes towards a first child may be different than for later children. Firstborn children have the full attention of parents and may be more indulged. This might explain why firstborns are more likely to be overweight or obese.
Women are more likely to quit smoking during their first pregnancy than during later ones, and firstborns are more likely to be breast-fed. The number of cigarettes smoked daily increases with birth order, starting in adolescence. Firstborns are five percent less likely to smoke than secondborns, and about thirteen percent less likely to smoke than fifthborns.
These and other differences could account for some of the explanation, but since information on family dynamics, as well as biological information, is lacking, it’s unclear how relevant birth order really is to your health at this time.
What can you do? Plenty…
Although you can’t do anything to change your natural height, and scientists are unclear as to whether IQ can be changed (as distinct from cognitive training interventions aimed at improving specific mental abilities), our personal decisions are key when it comes to our health.
What I mean is that an awareness of health problems or negative behaviors you may be more prone to from your birth order allows you to focus on ways to mitigate them.
For example, you can achieve better health by improving your diet, exercising, controlling stress, and engaging in good sleep habits, as well as eliminating harmful behaviors such as smoking.