Parental Investment

Robert Trivers parental Investment theory: 1972

• argues that the difference between male and females have their origins in the different amount of parental investment made by males and females
• human males investment is small- large amounts of sperm and remains fertile throughout life so is capable of many matings and is only limited by the number of female partners available- this takes little time and energy
• to maximise reproductive success it is best to have many matings with multiple female partners    

• females investment is substantial- she has limited supply of eggs and is fertile for around 30 years limiting total number of offspring produced
• pre-natal investment is large- 9 months and care fro years afterwards 
• best chance of reproductive success is to ensure survival of offspring and is choosier in the mates she picks 

• Helps us to understand mate preferences and to explain Buss's study
• Buss (1989)-cross cultural study of 37 cultures
  • women valued material resources and industriousness in potentail partners.
  • males who make less parental investments will be more reproductively successful if they have multiple matings wiht young fertile females.
  • this helps us to understand why youth is universally important to men
• His theory tells us little about homosexual relationships which are non-reproductive-
  • struggles to explain hwy lesbians are reluctant to engage in short term, uncommited sex when risk of reprodcution is not a threat
  • struggles to explain preference for younger partners shown by older gay men 
  • Tooby and cosmides (1992) modular mind explanation argue sthta the same principles can be used to understand relationships and that age preferences shown are inbuilt in men regardless of sexuality
• social and cultural changes that have dramatically affected parenting patterns, such as the increasing number of females in the workplace and the greater sharing of the parental role. These are largely ignored by the evolutionary approach, which is also impossible to test experimentally and which tends to make non-falsifiable statements.

The evolutionary approach states that we choose mates in order to reproduce and on the basis that females require resources and support, while men want to spread their genes as widely as possible and so look for reproductive potential and invest less in the parental role.

• This is heavily determinist and there are obvious counter examples; couples where the man is much younger, men who are monogamous and faithful, women who are unfaithful, heterosexual couples who have chosen not to have children, homosexual couples, stepparents who love their stepchildren etc.