Written by Matthew Sabatine
The following post/essay represents the views of only the author and not everyone at Common Issues.
Parental investment theory is something you can relate to if you are a parent; you sacrifice a lot of time, energy, materials, money, etc. to take care of your children when you could have expended that stuff elsewhere. But you saw your children as more worthwhile. Why? Did you expect some recompense from your child? Probably not. That sounds rather unreasonable: “Dear son or daughter, when you get the chance, you owe me food for all the times I fed you when you couldn’t feed yourself. You owe me diapers for when I had to change yours and you couldn’t change them yourself. You owe me money for all the stuff I bought for you since you were birthed, etc.” Many parents may obligate their children to pay rent while living in their home, but any parent will readily tell you that they don’t do that to requite for everything spent during the first 18 years of the child’s life. Instead, a parent’s returns are gained from having their children live a good and healthy life enough to have good reproductive potential.
Whether you believe it or not, there are a lot of unconscious forces at work, determining whether you decide to have sex. I believe that concern for parental investment is one of those forces. Your brain and your gut give you presentiments on, not just the possibility of an ill-conceived pregnancy, but also many other things that could result from completing the delicious deed. Studies on this trace back to the days of Robert Trivers in the 1970s.
Trivers’ model predicts that as the potential for parental investment increases, the finickier you’ll be about who you choose as a mate, but women will be finickier than men, because their minimum risks are higher. Previous studies helped to validate this, but the “curious exception” was that men were found to be less discriminatory during sexual experiences that didn’t require commitment and weren’t long-term (one-night stands) than they were during non-sexual relationships and encounters (single dates). A report from 2004 discusses a study on “mate choosiness in the context of five types of relationships that reflected explicitly defined, increasing levels of risk of parental investment for both males and females.” 468 undergraduate students, largely between 18 and 24, were given questionnaires discussing “29 personal characteristics” and asking them to rate “their minimum requirements” for a potential partner. While those results seemed to corroborate parental investment theory’s major predictions, it appears that perceived risk instead of real risk determine the sex differences in mate discrimination. 
I like to think that my watching men and women interact on social media for the past ten years has given me a bird’s eye-view of the differences between men’s and women’s sexual desires. I witness both sexes trying to accuse each other of being more salacious than the other, even though the scale seems to lean further toward men no matter how much the both sexes try to deny it. More men appear to be pestering women with sex offers than women are pestering men.
Since sex is linked to parenthood, obviously, I think parental investment theory is relevant to this. Men and women have different sexual strategies. Why do these differences exist? Throughout the course of evolution, men and women have developed different psychological strategies for navigating the adaptive problems unique to their sex. 
In the mammalian world, males are uncritical and un-methodical when choosing a mate, as supported by the fact that they can get enormous reproductive success by spitting their seed into many females. However, just one sexual encounter calls for more investment for females than males, due to the amount of time, resources, and energy required for gestating and rearing the resulting progeny. Considering this, men ought to be more empathetic to women when getting rejected for unsolicited casual sex. 
Selective mating strategies were necessitated among ancestral females because of great pregnancy possibilities and the resulting burdens. Humans have an immaturity period that outlasts the immaturity periods of plenty of animals in their world, and this was realized by us as early as the ancient days when women satisfied their reproductive interests best by choosing a man who could pass on good genes (as evinced in his attractive facial symmetry) and make himself available for continued investment in her and their kin. 
Professionals have employed a lot of research to know what standards and conditions human males and females use to evaluate each other. A study from 2001 on physical appeal looked at 51 male-female dyads who met for the first time and whose six-minute interactions were secretly filmed. It was found that the physically appealing women’s interactions achieved more excellence than the interactions of the women who were less physically appealing. The extroverted men had better interactions than the less extroverted men. 
A report from 1999 revealed that ovulating women, who are at the apex of their menstrual cycle, are drawn to the aroma of men who have superior bilateral symmetry in their body. Women using hormone-based contraceptives and in their low-fertility point lack this lure and interest. 
A 2002 study on heterosexual personal advertisement in Brazil found younger women being favored above older women and older men being favored above younger men. 
Genetic traits, such as major histocompatibility complex (the HLA complex accommodates the immune system in segregating the body’s own proteins from foreign proteins of invaders such as viruses and bacteria) has been shown to be relevant in mate choice. A 1997 study was done on the Hutterite people, where it was found that humans can have a strong bias against others who have the same HLA haplotypes as one’s own. 
A man’s economic status could influence a woman’s mating choice, if a man’s landownership and resources have any correlation with a woman’s reproductive success, based on Monique Borgerhoff Mulder’s 1990 study on the Kipsigis people of Kenya. 
For the sake of men’s reproducibility, throughout evolutionary time, men had to ensure their children got their imperative support to survive long enough to then also procreate. However, men’s investment didn’t require the same amount and duration as women. Thus, males’ and females’ reproductive interests have clashed due to such investment-inequalities despite having the same offspring-to-adulthood goal. 
Geary’s study in 2000 found that, despite some males’ great contribution to parental investment, others give little to none after their provision of gametes in the beginning. Some research has revealed that fathers in many cultures scored low on care-giving efforts. However, United States-based studies found that some fathers undertook childcare but significantly less than the mothers. Thus, there is a wide-ranging pattern of maternal care outweighing paternal care from birth to post-birth stages. 
Notwithstanding the costs that a woman faces when having a child, her responsibility for gestation and rearing makes her certain of maternity. Since a man only donates sperm, he can’t be as certain of paternity. A woman’s libido culminates during ovulation, but her ovulation is hidden, so that a man doesn’t know when coitus will result in pregnancy. It is a real concern among today’s men to accidentally become the social father to another man’s genetic ankle-biter(s), supported by the statistic that 2-30% of all births involve this accident. Traditional societies and our ancestors likely had the same issue. A woman, on the other hand, may likely fear another woman stealing her mate’s investment, but can’t likely be tricked into bringing up a child she believes belongs to her when it really belongs to another woman. 
Sex differences among men and women can be evinced when observing their different reactions to betrayal and unfaithfulness. Verbal statements and physiological stimulation were gauged when asking study participants about how they felt while imagining their long-term partner either (a) having casual coitus with another person or (b) making an intimate emotional tie with another person. Study results had revealed that women are more unhappy, disappointed, and worried by a partner’s emotional disloyalty and men are more troubled by a partner’s sexual disloyalty. 
To reiterate about women’s unconscious unawareness of investment inequalities, we can see this again when we look at the concept of anisogamy and the hugely different sizes of gametes (sex cells) between the two sexes. A female makes roughly 500 ova, and if a man’s number of gametes equates to his number of neurons, he can make 86 billion gametes in one month, meaning there is a 1:100,000 cost of sperm compared to ovum. All the resources provided to the zygote, to develop into a newborn that will outweigh its zygote size by one hundred billion times, come directly from the mother and not the father. 
Simon Hampton’s book Essential Evolutionary Psychology illustrates parental investment on a r-K spectrum, wherein the r side represents those investing none or little in progeny. Those on the K side are investing much. Since those on the r side generate numerous progenies, few will survive, and therefore that side of the spectrum can’t be concerned with quality of care as much as quantity of progenies. Those on the K side deliver few progenies, and many more can survive, probably due to the obvious fact that parents are able to invest more time, energy, focus, and resources on those few. Those few involved with the K side are born comparatively immature, weak, and helpless, endure an extensive pubescence, and therefore require a lot of parental investment. 
It is believed that the size of a human infant’s head, less than 4 weeks old, in comparison to the female pelvis size, dictates a length of gestation shorter than what might be best. Neonates are so defenseless and exposed that we might see gestation as being prolonged in the first post-birth months. As humans are such a K-selection species (heavily investing in fewer progeny that are born so overwhelmingly immature), such aggressively concentrated investment between newborn and caregiver produces one of the most intimate, peaceful, and nurturing images of human interaction. Most prominently, the image is of a mother and child instead of the father and child. 
Discrediting the k/K Selection Theory
It has been brought to my attention that some believe the r/K selection theory has become “marginal” and discreditable among professionals and isn’t adequately applicable to life-history strategies. It appears that it could be disliked due to its misuse among “proponents of eugenics and racialism”, as pumpkinperson.com would explain it. Jean Phillipe Rushton (1943-2012) is reputed to be that racialist stain on r/K selection theory’s usefulness history because of his using it to explain the cognitive and behavioral disparities between Africans, Europeans, and East Asians. 
Empirical studies of the 1990s impugned r/K selection theory after it was popularly used as a heuristic device in the 1970s and 80s. Stephen C. Stearn’s critique reportedly “drew attention to gaps in the theory, and to ambiguities in the interpretation of empirical data for testing it.” “He concluded that r/K theory was a once useful heuristic that no longer serves a purpose in life history theory.” Though r/k selection is said to be supplanted by newer models, many of its important themes are still heuristically used.  Therefore, it seems apparent to me that I am not obligated to abandon Simon Hampton’s use of it completely, but also I am not warranted to rely on it heavily. The flaws of r/K selection theory and my probable mistakes of this post can be discussed in a later post.
About the author: Matthew is interested in discussing social psychology, neuroscience, anthropology, sociology, human biology and anatomy, mental health disorders, philosophy, the psychology of religion, and the history of religion. Matthew loves his friends, his family, and his dog named Sampson. You can contact Matthew at firstname.lastname@example.org.
Disclosure statement: I am not a licensed therapist nor doctor. My intention is to not pretend to be either. The information contained in this article is not meant to be accepted instead of a doctor or licensed therapist’s advice. All information contained herein is based on my interpretation of the books and articles I read. My hope and desire is that any troubled person reading this would feel encouraged to get help from a licensed practitioner.
 Hampton, Simon, Essential Evolutionary Psychology, pages 97-99, SAGE Publications, first published 2009, Print.