Original Article Do grandparents favor granddaughters? Biased grandparental investment in UK ☆,☆☆ Antti O. Tanskanen a, ⁎ , Anna Rotkirch b , Mirkka Danielsbacka c a Department of Social Research, University of Helsinki, Helsinki 00014, Finland b Oxford Centre for Population Research, Department of Social Policy and Intervention, University of Oxford, Oxford OX1 2ER, UK c Department of Philosophy, History, Culture and Art Studies, University of Helsinki, Helsinki 00014, Finland Initial receipt 21 September 2010; final revision received 24 February 2011 Abstract Differential grandparental investment in grandchildren is often explained with paternity uncertainty. The asymmetric inheritance of the sex chromosomes, especially of the X chromosomes, may also bias grandparental investment. Recent studies show that ignoring the sex of the grandchild can mask important differences in the investment patterns of the same grandparent category, but this has not been tested in contemporary societies with nationally representative data. With 17 variables from the Involved Grandparenting and Child Well-Being 2007 survey, we tested differential grandparental investment as reported by British and Welsh adolescents and compared predictions based on X- chromosomal relatedness with predictions based on paternity uncertainty. The theories are expected to differ with regard to grandmaternal investment in grandsons and granddaughters. We test whether paternal grandmothers invest (H1) more in granddaughters than in grandsons, (H2) more in granddaughters than maternal grandmothers do and (H3) less in grandsons than maternal grandmothers do. In addition, following the suggestion that paternal grandmothers may reduce sibling competition between girls and boys by harming grandsons, we study whether (H4) paternal grandmothers channel more noninvestment into grandsons than into granddaughters. The results show no convincing support for the type of sex discrimination of grandchildren that is predicted by X-chromosomal relatedness theories, but do provide support for the paternity uncertainty theory. X-chromosomal relatedness does not appear to shape grandparental behavior in developed societies. © 2011 Elsevier Inc. All rights reserved. Keywords: Grandparent; Grandchild; Sex chromosome; Paternity uncertainty 1. Introduction According to kin selection theory (Hamilton, 1964a, 1964b), grandparents may increase their reproductive success by investing in grandchildren, with whom they are on average genetically related to a degree of 25%. Grandparental investment may be understood as all actions and characteristics of grandparents that increase the fitness of a grandchild while detracting grandparents from other resource spending related to reproduction, survival, devel- opment and maintenance. Compared with parental invest- ment, grandparental investment costs are typically lower, especially for postreproductive individuals, while its poten- tial fitness benefits are high (Clutton-Brock, 1991; Euler, in press; Trivers, 1972). In traditional and subsistence societies, kin investment is usually measured as the effect of grandparental presence on child mortality (Mace & Sear, 2005; Sear & Mace, 2008). In developed societies with low infant and child mortality, grandparental investment is measured through variables assumed to improve the grand- child's well-being and security such as contact frequency, emotional closeness and financial assistance, as reported by the grandchildren, by their parents or by the grandparents themselves. Close grandparental contact appears to be especially important for adolescent coping in contemporary Western societies (Attar-Schwartz, Tan, Buchanan, Flouri, & Griggs, 2009). Studies from both traditional and developed societies and using a wide range of variables show that grandparental Evolution and Human Behavior 32 (2011) 407 – 415 ☆ All coauthors have seen and approved the submitted article. ☆☆ The research was supported by an individual research grant from Kone Foundation (Rotkirch) and is part of the research project Baby Boomers' Generational Transmissions in Finland (Gentrans) at the University of Helsinki. ⁎ Corresponding author. 1090-5138/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.evolhumbehav.2011.02.001