Rhetorical Question: Does birth month affect left handedness?

Scientists have figured out how  Temperature-Dependent Sex Determination affects the gender of some reptiles.  If the ambient temperature of a nest is above or below a certain degree, or the temperature is inside or outside a certain range, all the eggs in the nest will be the same gender, either male or female.  Climate change is already playing havoc with sea turtle populations.

I have often wondered it ambient temperature has a similar effect on humans.  A study at the University of Vienna showed a disparity of handedness depending on birth month.  (The original paper can be found here.)  More children were born from November to January.  The authors of the study ascribe it to hormones, but I wonder if the ambient temperature during the first trimester plays a role.  A child born in January was conceived in April, and the foetus spent the third and fourth months in the hottest part of the year.

Early in 2019, I polled members of a left handers’ group on facebook, asking people to state their birth month and which hemisphere they were born (north or south).  Over 350 people answered.  After accounting for hemisphere, the results were similar to the U of Vienna study.  It was voluntary reporting, but I doubt people had any reason to lie (other than worrying about facebook’s data collection).

First, the number of people born in each month, reversed north/south seasons accounted for:

  • January: 48
  • February: 41
  • December: 33
  • November: 31
  • July: 29
  • September: 28
  • June: 27
  • March: 26
  • May: 23
  • April: 23
  • October: 22
  • August: 22

The monthly average was 29.41666…, and the standard deivation was 7.6752669153732. The empirical rule for normal distributions is:

  • within 1σ = 68%
  • within 2σ = 95%
  • within 3σ = 99%

Ten of the twelve months fall within 1σ which should be only 68% of the people, but turned out to be 75.4%.  February is off by 1.51σ, which should account for 85%, and was actually 86.7%. Close enough.

January is off by +2.42σ, which should account for 98-99% of people. That barely falls within the range of normal, but close enough to merit further study.


  1. dangerousbeans says

    What is the birth month distribution ignoring handedness? Maybe more people in general are born December through February

    • says

      I lost the original data set, but there were only two people from the southern hemisphere. Both gave their birth months as September or March (I forget which), which didn’t affect anything. December-June or January-July would have made a difference.

  2. says

    Since internal body temperature of humans is very, very tightly regulated and does not generally vary much with weather, I do not think that temperature has an influence on how many left handed people are born.

    However with increased latitude changes the variation of daytime length throughout the year, and that has demonstrably influence on the inner chemistry of humans (most obvious being vitamin D production) and thus hormones are in my opinion more likely culprit, as per the study.

    If there is a correlation between the rate at which left-handed people are born and the temperature, then in my opinion it is correlation caused by a common cause – sunlight – not correlation due to direct causation of temperature causing left handedness.

    This is of course just playing with numbers and speculating, not exact scince:
    I have run your dataset through Minitab (I took average month temperatures for CZ, as it is typical for moderate climate and real temps for your dataset are not available) and calculated correlation between the temperature and the amount of future left-handed people when the were in 1, 2 etc month of pregnancy at that month. I only looked at straight linear correlations. Out of 9 thus calculated correlations three were statistically significant and meaningful:
    3 month of pregnancy: Rsq=47,25%, p=0,014, positive correlation
    4 month of pregnancy: Rsq=39,79%, p=0,0,28, positive correlation
    Month of birth: Rsq=44%, p=0,018, negative correlation

    So if daylight time influences the rate of left handedness, then it would icrease it somewhere between at the end of first and beginning of second trimester like you say. The apparent “decrease” at birth is merely caused by the 6 months cycle in the temperature and is a reverse of the 3rd month correlation. Any positive correlation with mothly temperature will turn negative when shifted 6 months in either direction

    Your data are normally distributed at 95% CI – p=0,052, where H0 assumes normal distribution.

    At a glance, these numbers do NOT follow the general monthly birthrate variation in EU (again, I took that, because it was available and the full origin of your dataset is unknown). That only varies about 15% between the highest and lowest birth-rate, and peaks in summer, whereas yours left-handed births peak in January and drop to 50% of that mid-summer.

    • says

      There’s also the fact that *if* heat were a factor, those in tropical latitudes would have higher rates of left handedness and countries north of a certain latitude (e.g. 50N) would have lower rates. I have looked but found no studies on this.

      Gender is a common factor. I didn’t take note of the lefties in the facebook group, but most studies on handedness show higher rates in XY people than XX, and not because scientists are excluding women like they do with medical trials.