Women make greater investment through pregnancy and post natal breastfeeding, this is extended due to adaptive pressures making the brain larger therefore the child is born more immature compared to other animals.

Women are more mature than men therefore invest more in their children. Simpson et al found that because women produce far fewer eggs than men produce sperm they are more mature in their choice of mate therefore invest more.

Costs of childcare are more for women. They have a minimum 9 months carrying the child in addition to years of aftercare. Symons stated that minimum investment for men is a few moments of copulation and a teaspoon full of semen making random mating more costly for females.

Greater investment by females can be explained through parental certainty. Because the woman is carrying the child they can be sure they are the true parent but men have the risk of investing in another man's child.

Sex differences in jealousy state that because women are more jealous of the shift in emotional focus in terms of infidelity, they invest more in the child.

Strength

Weakness

Methodalogical issues- Social desirability bias as they used a survey.

Gender bias- Alpha- differences maximised.

Methodalogical issues- Population validity.

Benefits- additional social support, perhaps higher quality genes.

Benefits- child bonding with two fathers, more options when in need of help

Risks- abandonment, male retention strategies e.g. violence against female or other male

Risks- lower quality genes, end of relationship, father having no bond with child

Free will vs. Determinism. This study is too deterministic- doesn't allow free will. Not all men invest.

Gender bias- Alpha bias- differences maximized.

Are males biologically less prepared to invest? Michael Gove suggested it was influence from "lad mags" linked to a rise in feckless fatherhood however Geher et al found that male attitudes are shaped by biological forces. 91 non-parental undergraduate participants completed a parental investment perception scale and were exposed to parental related scenarios. There were no sex differences in self report responses however when costs of parenting were high ANS arousal in men heightened showing men are biologically less prepared to invest in children.

Evolutionary explanations are too reductionist. Rowe stated that basing all investment on evolutionary factors means that conclusions are severely limited. It reduces the complex behaviour of making investment down to evolution and doesn't take into account factors such as relationship with mother. Belsky claims that childhood experiences e.g. parental divorce correlate with men's investment with their children.

Parental investment theory would predict that investment by fathers would always be greater if they know the child is theirs. They would not want to spend time and resources bringing up another man's child however some studies have contradicted this. Anderson found that men did not discriminate financially between a step child in a current relationship, compared to their own child from a previous relationship. Though this may be a strategy to convince the mother that he is a good provider (by investing in the step child) and this in turn can promote future chances for reproductive success.