1. Sex Ratios: Human Twins and Fraternal Effects
Sarah C.L. Knowles, Ben C. Sheldon 
Current Biology 
Volume 17, Issue 18, Pages R801-R804 (September 2007)
DOI: /j.cub
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2. Figure 1
Historical records of Finnish families were kept in some detail by the Lutheran church in the 18th and 19th centuries.
Recent work [8] shows that the sex composition of twins had important effects on their reproductive fitness (image courtesy of V. Lummaa).
Current Biology  , R801-R804DOI: ( /j.cub )
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3. Figure 2
Effect of sex composition on fitness components for human twins born in Finland in the 18th and 19th centuries (white bars females, grey bars males).
Female twins that had a male co-twin were both less likely to marry (A) and produced offspring at a lower rate even if they married; this resulted in lowered lifetime fecundity (B). Reproduced, with permission, from [8] Copyright (2007) National Academy of Sciences, U.S.A.
Current Biology  , R801-R804DOI: ( /j.cub )
Copyright © 2007 Elsevier Ltd Terms and Conditions