Using a large sample of non-institutionalized individuals from the NetherlandsHamilton (1964) argued that selection should favour investment in closely related kin, and this hypothesis has been largely confirmed in a wide variety of studies. A reasonable precondition for investing in someone is knowing whether they are alive or dead, and so whether someone has this knowledge is a plausible proxy for openness to investment. More specifically the authors say "Our hypothesis is that respondents will be more likely to know whether a sibling is alive when they are fully related than when they are not."
(n = 7610), we examined the influence of relatedness on an individual’s knowledge about whether their sibling is alive or not. Respondents were generally less likely to know whether their sibling was alive if they were not fully related. The effects were stronger for differences between paternal half-siblings and full siblings than for differences between maternal half-siblings and full siblings.
This study is part of a larger, ongoing, study of kinship in the Netherlands (the Netherlands Kinship Panel Study (NKPS)). For the present research "respondents [were] asked to list all their siblings (adopted, step, half- and full siblings) and to indicate whether they know whether this sibling is alive or not".
As the authors note, an alternative view is that levels of affiliation and dispostion to invest might follow development, in particular who was around when an individual grows up. "This proximate theory would suggest that relatedness in most families is confounded with being raised together." They proposed to track this confound by distinguishing both paternal half-siblings (who are generally raised apart) with full siblings and maternal half-siblings (who are generally raised together) with full siblings. The study excluded adopted siblings because there were so few in the study. (It would be interesting to know more here, but a specific study would probably be needed to ensure enough adopted individuals for useful analysis.)
Data analysis considered up to the first six siblings, and controlled for "education (9 categories: from incomplete/primary to university/postgraduate; treated as interval), age, age differences between the siblings, gender of the respondent, and number of family transitions before age of 16" where a "family transition" was "an alteration in the respondent’s family living situation since birth, for instance going to live with grandparents or with another family member". The analysis constructed multinomial logistic regression models to calculate the "independent effect of sibling type on knowledge about the sibling’s death". I was surprised to hear that over 1200 of the subjects (total number 7610) even had as many as six siblings.
Figure 1 below shows the raw proportion of respondents who did not know whether a sibling was alive or dead as a function of sibling type and number.
The analysis using the regression models showed that across "all six models, sibling type proves to be a highly significant predictor of knowledge about a sibling’s death [...]. The other variables did not prove consistent predictors." And the predictive value of sibling type is dramatic: "respondents are 25.75 times more likely to not know whether their sibling 1 is alive when" that sibling is a step-sibling than when he/she is a full sibling (emphasis added).
The authors are careful to note that their hypothesis concerned ultimate outcomes, rather than proximate mechanisms. It's an interesting question by what means relatedness is tracked, and to what extent it depends on epigenetic factors. Surprisingly the authors don't mention a study published in 2007 in Nature regarding sibling detection. That paper is well worth a look - it doesn't settle the questions here, partly because the proposed system is partly dependent on observed interactions with mothers, and duration of coresidence with candidate 'siblings', but it should have been discussed. Also, the conclusion of the Nature paper is worth quoting in this celebratory article:
"These results contribute to a growing body of findings showing that humans are not immune to the evolutionary forces that have shaped other species, and that Darwinism has a central role in discovering the neural and psychological architecture of our species."
Hamilton, W.D. (1964). The genetical evolution of social behaviour I, II. Journal of Theoretical Biology, 7, 1-52.
Lieberman, D., Tooby, J. & Cosmides, L. (2007) The architecture of human kin detection, Nature 445, 727-731.
Thomas V. Pollet, Daniel Nettle (2009). Dead or Alive? Knowledge about a Sibling’s Death Varies by Genetic
Relatedness in a Modern Society Evolutionary Psychology, 7 (1), 57-65 DOI: http://www.epjournal.net/filestore/ep075765.pdf