Because of concealed ovulation, internal fertilization, and female infidelity, parental certainty is asymmetrical: unlike females, who are (with exception of rare maternity-room mistakes) always certain of maternity, males can never be certain of paternity. Further, during our evolutionary history, females would have benefited from 100% certainty of maternity. Current estimates of extra-pair paternity (paternity by the non-domestic father, or cuckoldry) are between 1 and 30%, with the best estimate at about 10% (Baker & Bellis, 1995; Cerda-Flores et al., 1999; Neale, Neale, & Sullivan, 2002; Sasse et al., 1994; Sykes & Irven, 2000). In other words, approximately 1 in 10 children are the product of female infidelity. This asymmetry in parental certainty has contributed to an asymmetry in human parental investment (Bjorklund & Shackelford, 1999 Geary, 2000). As a consequence of having to carry a child to term, females invest more in and provision more for children than do males. Additionally, if a female nurses her offspring she could be bound to a minimum of 1.5–2 years of further parental investment that is not shared by males. There are two reasons as to why parents would invest in offspring. The first is to increase their own genetic fitness (e.g. increased number of their genes in future generations) and the other is to influence their relationship with their offspring's other parent (Anderson, Kaplan, & Lancaster, 2001).