Does the Fraternal Birth Order Effect Influence Handedness?

The fraternal birth order effect (FBOE) has been linked to male sexual orientation, with evidence suggesting it applies only to right-handed men. Some research suggests left-handed men do not show this pattern. However, the FBOE is often confounded with the female fecundity effect (FFE), which complicates interpretation. Prior research has shown that mothers who are more likely to have sons with same-sex orientation may also be more fecund, making it hard to distinguish FFE from FBOE. This overlap has led to uncertainty about whether the FBOE is a real phenomenon or an artifact of other factors. No prior work had resolved how these effects interact with traits like handedness. That uncertainty drove this study to explore whether FBOE might influence handedness in a similar way. This gap motivated the use of new analytical methods to disentangle FBOE from other confounding variables.

The study aimed to investigate whether the FBOE could be observed in handedness, a trait with known sex differences. Researchers sought to determine if the FBOE, known for sexual orientation, also applies to handedness in men. They also wanted to test whether the FBOE is confounded with other factors like the FFE or parental age. The study focused on distinguishing FBOE from FFE using recent analytical methods. The researchers proposed that if FBOE exists for handedness, it should show similar patterns as in sexual orientation. They also wanted to assess whether parental age might act as a confounder in FBOE analyses. This study tested the FBOE in a new context to better understand its validity and limitations. The goal was to clarify whether the FBOE is a genuine effect or a statistical artifact.

The researchers applied recent analytical methods to assess the FBOE in handedness. They used Khovanova's technique to examine the binary trait of handedness in relation to birth order. This method allowed them to compare men with one older brother to those with one younger brother. They also controlled for parental age in separate models to assess its influence. The study tested multiple effects simultaneously, including FFE, paternal age, and birth order. They analyzed data from both men and women to compare patterns. The researchers used statistical models to isolate the FBOE from other variables. These methods allowed them to determine whether the observed effects were due to FBOE or other confounding factors.

A straightforward application of Khovanova's method showed a FBOE-like pattern in handedness for men with one older brother. The ratio of right- to left-handed men differed between those with an older brother and those with a younger brother. However, this effect disappeared when parental age was controlled for. Models including multiple factors found significant female fecundity effects on handedness. Paternal age also showed a significant effect on handedness in men. No FBOE was found in these more comprehensive models. Women showed different patterns, with no fecundity or parental age effects. Birth order and sibling sex still influenced handedness in women. These findings suggest that FBOE may not be a robust phenomenon when other variables are considered.

The authors concluded that factors influencing sexual orientation in men may also affect handedness. They noted that parental age is a potential confound in FBOE analyses. The FBOE observed in prior studies may be an artifact of uncontrolled variables like parental age. The study found no evidence of FBOE when multiple factors were considered. Female fecundity and paternal age effects were significant in men's handedness. Women showed distinct patterns with no fecundity or age effects. The researchers proposed that some analyses of FBOE may overlook important confounding variables. These findings suggest that FBOE may not be as strong or consistent as previously thought.