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: 10.1016/j.cub.2007.07.027 Copyright © 2007 Elsevier Ltd Terms and Conditions

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 2007 17, R801-R804DOI: (10.1016/j.cub.2007.07.027) Copyright © 2007 Elsevier Ltd Terms and Conditions

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 2007 17, R801-R804DOI: (10.1016/j.cub.2007.07.027) Copyright © 2007 Elsevier Ltd Terms and Conditions