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, 5 (4), 524-7

Humans at Tropical Latitudes Produce More Females


Humans at Tropical Latitudes Produce More Females

Kristen J Navara. Biol Lett.


Skews in the human sex ratio at birth have captivated scientists for over a century. The accepted average human natal sex ratio is slightly male biased, at 106 males per 100 females or 51.5 per cent males. Studies conducted on a localized scale show that sex ratios deviate from this average in response to a staggering number of social, economical and physiological variables. However, these patterns often prove inconsistent when expanded to other human populations, perhaps because the nature of the influences themselves exhibit substantial cultural variation. Here, data collected from 202 countries over a decade show that latitude is a primary factor influencing the ratio of males and females produced at birth; countries at tropical latitudes produced significantly fewer boys (51.1% males) annually than those at temperate and subarctic latitudes (51.3%). This pattern remained strong despite enormous continental variation in lifestyle and socio-economic status, suggesting that latitudinal variables may act as overarching cues on which sex ratio variation in humans is based.


Figure 1
Figure 1
(a) Visual depiction of average annual proportions of males at birth for individual countries around the globe from 1997 to 2006. (b) Colour-coded average proportion of male offspring for each five degrees of global latitude, for 202 countries ranging from 0° to 65°. (c) Mean (+s.e.) percentages of males at birth for countries categorized according to continental location. Human sex ratios showed significant continental variation. (d) Mean proportion of males at birth (s.e.s and s.d.s were too small to plot) for countries located in tropical, temperate and subarctic latitudes. Countries at tropical latitudes produced significantly less males compared with temperate and subarctic latitudes (F=11.07, p<0.0001, tropical–temperate and tropical–subtropical comparisons, p<0.01). All sex ratios were calculated using the average for 1997–2006. All figure segments are colour-coded (blue, more than 51.7% males; yellow, 51.2–51.7% males; orange, 50.7–51.2% males; red, 50.7% or less males). In (c) and (d), significant differences are indicated by different capital letters.

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