Motivation and emotion/Book/2018/Parental investment theory

Overview


Imagine that you are a lady sitting in the food court eating your lunch when something picks up your attention. A young man walks up to a mother and her baby in a pram at the table opposite you. You find out that he is a brother as he greets her saying "hey sis" and starts to give the baby attention by smiling and continuing to communicate with the baby. Now imagine a different scenario where her brother said hello to the baby but didn't give him any more attention? Now if her brother comes up and says that you caught his attention and asks to go on a date with you, would you accept? You would probably be more willing to accept the date if the brother had been smiling and playing with the baby. This is exactly what Guéguen (2014) found in his field experiment, whereby females were more likely to accept the brother on a date if he gave the baby lots of attention, for example, as shown in Figure 1. This is further explained by Trivers' (2002) parental investment theory, which states that females are more attracted to males who are willing to invest in child-rearing. The following chapter explores parental investment theory and what it suggests about our motivations behind seeking romantic and sexual partners.

History
Before delving straight into parental investment theory, it is important to understand how it was developed. There are two main frameworks that Trivers (2002) mentions throughout his paper on parental investment theory. The first being Darwin's sexual selection theory from 1871, and the second being from Bateman's three principles from his 1948 study.



Charles Darwin's sexual selection theory


Have you ever noticed that some species display a common characteristic compared to other species in the same habitat? This can be explained by Darwin’s sexual selection theory, as it suggests that species develop certain traits due to having an advantage over other mates in the past. This means that these males have been more successful at attracting female’s and have had higher mating success rates relative to other competing males (Hosken & House, 2011), such as in Figure 2. These traits are often referred to as colours or ornaments of an animal, however in humans these traits are often compared to social behaviours and personality (Kenrick, Sadalla, Groth & Trost, 1990). Moreover, Darwin noticed two mechanisms in his theory, the first being that males often compete with other males to possess a female, and the second being that females were often more selective at choosing a male (Hosken & House, 2011).

Angus Bateman's three principles
In 1948, Bateman did a study on fruit flies looking at the success rate of males versus females by placing five of each gender in the same space, such as in Figure 3, and found that females were more likely to be successful in making offspring compared to the males (Trivers, 2002). As such, three principles were concluded from this study and other studies that replicated it, which are displayed in Table 1. For more information about the evolution of mating, see motivation and relationships (Book chapter, 2013).

Robert Trivers' parental investment theory


With these frameworks in mind, Trivers (2002) developed the parental investment theory which can be defined as the amount of investment a parent gives to a child in order for them to survive and reproduce in the future, whereby this investment requires a lot of energy from the parents. Therefore, overtime, they have less energy to invest in more children. As such, when Trivers (2002) talks about investment he means not only the energy it take to produce a child, but also the energy needed to look after the child, such as giving them food, care and attention. Trivers (2002) helps explain the gender difference of sexual selection theory through his theory of parental investment as females are more likely to invest in a child as they are the ones who carry the baby in the womb and nurture them through infancy (Kenrick, Sadalla, Groth & Trost, 1990), therefore, they are more selective in choosing a potential partner. This is because they do not have much energy to invest in lots of mates. On the other hand, Trivers (2002) stated that those who invest less would be more competitive, as they are competing for females who are fertile and available to produce offspring. Trivers (2002) also argued that the number of fertile females available in space and time would predict the competitive nature of males, as males would compete depending on how available a fertile female is.

Humans are a pair-bonding species, in which high male parental investment is the norm. Because of that, men and women are equally selective in long-term mating, since men then invest heavily in the offspring. As women have the higher obligatory biological parental investment, they are attracted to men who will be able to make high non-obligatory investments in the offspring. Therefore, they are attracted to men with resources and that are kind towards children. Men, on the other side, are attracted to women that are young and physically attractive, since these are signs of fertility.

Defining romantic and sexual attraction
Before being able to apply parental investment theory to attraction, it is important to briefly distinguish between the two types of attraction. As such, romantic and sexual attraction have been used interchangeably throughout the literature and have often been referred to as the same type of attraction. Moreover, those who experience sexual attraction often experience romantic attraction and vice versa, and, individuals often have their own definition of each type of attraction (Lund, Thomas, Sias & Bradley, 2016). Even so, there will be a brief definition of each attraction type. Throughout the research, authors often label attraction as an umbrella term for both romantic and sexual attraction, and often define romantic attraction as a type of sexual attraction as well. For more information about the different kinds of love, relationships and sexual desires, see love and lust (Book chapter, 2015).

Romantic attraction
Lund, Thomas, Sias and Bradley (2016) distinguish between romantic and sexual attraction. As such they describe romantic attraction as the desire to form relationships that are meaningful and emotional. For example, someone may want a romantic relationship but are not interested in a sexual relationship. This type of attraction may be tied to our interpersonal need of belonging and being cared for.

Sexual attraction
On the other hand, sexual attraction is what motivates us to be attracted to others due to our sexual desires, fantasies and wanting to have a sexual encounter with that person (Lund, Thomas, Sias & Bradley, 2016). This type of attraction is what motivates us to choose an appropriate partner to produce offspring with (Fisher, Aron, Mashek, Li & Brown, 2002). Sexual attraction is often experienced with romantic attraction as well.

Parental investment theory and romantic and sexual attraction
If romantic and sexual attraction motivate forming relationships and having sexual encounters with others, then what does parental investment theory suggest about our motivation to be attracted to one person yet not another? This section discusses what parental investment theory suggests about our motivations to seek romantic and sexually attractive partners.

Physical attraction


Physical attraction is observable, but is everyone attracted to someone depending on their looks? It seems to help, but this is not always the case according to parental investment theory. Females are more likely to sacrifice "hot looks" for someone who can protect and invest in a future child (Frisby, Dillow, Gaughan & Nordlund, 2010). However, for males, physical attraction is important because young females are an indicator of a mate who can successfully produce offspring (Meltzer, McNulty, Jackson & Karney, 2014), as shown in figure 5. Because physical attraction is related to being motivated in finding a partner to produce offspring with, perhaps it is more of a sexual attraction rather than a romantic one.

Castro, Hattori, and Lopes (2012) wanted to know whether males and females choose partners in relation to "traditional sex preferences", in this case in terms of parental investment theory, or whether they choose partners who are similar to them. To test this idea, 267 young Brazilian university students were recruited, with 120 who were in a romantic relationship and 147 who were not currently involved in one. However, participants must have had a stable relationship for one year to continue on in the study. Castro, Hattori and Lopes (2012) gave out individual questionnaires to each participant to measure their self-evaluation and their perception of one's most recent romantic partner across a number of traits on a Likert scale. The traits that were included will be found in Table 2.

What Castro, Hattori and Lopes (2012) found was that males, whether they were in a current romantic relationship or not, scored their partners as more physically attractive (in the face and body) compared to themselves, which goes in favour of parental investment theory, as males are said to choose amongst young and fertile females. However, they did find something that was not in favour of parental investment theory, as males scored themselves as healthier than their partners. Now, this is unusual because the theory would suggest that males should score their female partners as healthier since they are the ones to carry offspring, but this was not found in this study.

Additionally, the results from female participants found no difference between their self-evaluation and the evaluation of their romantic partner across traits (Castro, Hattori & Lopes, 2012), concluding that female participants rate their partners as being as physically attractive as themselves. According to parental investment theory, females should have scored their partners as having more resources such as a better financial status to be able to provide for a child. However, this was not found in the study probably due to the young sample of university students (Castro, Hattori & Lopes, 2012).

Gender difference in mate attraction
As discussed in accordance with parental investment theory, females are often choosier in their partners, whereas males are not so much, but are rather more competitive in obtaining a female (Trivers, 2002). But is this also true for human relationships?

As such, Kelley and Malouf (2013) were interested in testing whether there was a gender difference or age difference when selecting mates. They analysed ratings from two American newspapers which consisted of blind date columns. They examined 224 blind dates from one newspaper from 2007 to 2011 and examined 123 blind dates from another newspaper from 2009 to 2011. What they found was that females rated the dates less favourably than men, which had been consistent with previous findings where females were less likely to agree to give more contact details to males after a speed-dating event. Kelley and Malouf (2013) concluded that, because of this, females were seen as choosier in their dates than the males. However, they thought that younger females paired with older males would have better ratings, but this was not found in the study as ratings did not differ according to age. Parental investment theory would suggest that females should rate older males higher, and males should rate younger females higher (Kelley & Malouf, 2013).

There are no studies that have yet tested the competitive nature of males fighting for attractive females, perhaps because of the ethical issue surrounding this kind of study, as it could lead to fighting amongst individuals. To combat this issue however, males who offer protection and resources to females would have a higher chance of being picked as a mate than other males according to parental investment theory (Grigorovici, 2018). For example, Miner and Shackelford (2010) examined the literature on parental investment theory and sex differences in mate choice and found a recurring finding that females were more willing to have long-term relationships with partners who could provide the resources needed to invest in a child.

Social behaviours


Do our social behaviours matter in our motivation to find an attractive partner? Parental investment theory and research suggests that it depends on gender and whether the partner is seeking a long- or short-term relationship.

Guéguen (2014) was interested in males' social behaviour in relation to parental investment and how this behaviour could influence how females were attracted to males. He conducted a field experiment with 52 young females who happened to be sitting at a pavement area in between two bars, similar to the lady sitting in Figure 6. As such, three young male confederates in their 20s were recruited to be a brother. Two mothers and their babies were also recruited as confederates in the study and was the male's sister. As such, Guéguen (2014) set up two conditions, a parental investment condition and a control condition. In the experimental condition, the male waited for a young female to sit down near the pavement area, and not long after the male would sit down near her. Then, a minute later the mother and her baby would stroll past and notice her brother and greet him. Then the male confederate would greet his sister and also kiss the baby and interact with them. However, in the control condition, the male confederate was instructed to say hello to the baby but nothing else. After their two-minute conversation the mother and her baby departed, and the male then asked the young female for a date, by approaching her and saying that he caught her attention earlier. Guéguen (2014) found that the young females were three times more likely to accept the invitation if the male had interacted positively with the baby. In favour of parental investment theory, this shows that males' willingness to invest in looking after a child is an important factor in how a female judges the attractiveness of a potential mate.

Similarly, Bleske-Rechek, Remiker, Swanson and Zeug (2006) conducted two separate experiments that tested male and female’s attractiveness to an opposite sex and their interaction with a baby. In the first experiment, males and females viewed a picture of either the opposite sex interacting with the baby or ignoring the baby in distress. What they found was that females rated the male's attractiveness lower (in relation to a long-term partner) when the participants had viewed the picture of the man ignoring the baby. They also found was that males were affected by this, whereby when they saw the picture of the female ignoring a baby in distress they rated their attractiveness as a long-term partner as more negative as well. This study demonstrates that males are also sensitive to the behaviour of the females and how they treat children, as males need a caring partner to nurture their offspring.

In Bleske-Rechek, Remiker, Swanson and Zeug's (2006) second study, males and females either looked at pictures of an opposite sex interacting with a baby, ignoring a baby or doing domestic chores. Interestingly, females rated males at the same level of attractiveness when they interacted with a baby and did domestic chores, in this case vacuuming. They suggest that this is because vacuuming could be seen by females as a behaviour that helps the mother out with children (Bleske-Rechek, Remiker, Swanson & Zeug,2006). Additionally, they did not find a negative impact on males attraction ratings of females who ignored a baby in this condition.



Personality traits
Certain characteristics are also desired in others, but this depends again on gender and if the person is seeking a long-term or short-term relationship.

Miner and Shackelford (2010) examined a numerous amount of studies that have tested what males and females desire in long- and short-term partners. In particular, they found that males and females both want a partner who is caring, for example in Figure 7, and intelligent as long-term partners. And in the short-term they found that both males and females prefer a physically attractive partner, perhaps because the female does not have to rely on the male to stay and look after the child, and so would rather choose desirable genes than a male's investment in child-rearing (Bleske-Rechek, Remiker, Swanson & Zeug, 2006).

Short-term and long-term relationships
Griskevicius, Cialdini and Kenrick (2006) conducted multiple experiments on the effects of parental investment on partners creativity. Ninety one undergraduate participants were recruited and wrote a story about an ambiguous image for baseline purposes. The participants were then split into two groups. The experimental group was given a mating cue (viewing six attractive photos of opposite sexes) and were then told to write two stories on different ambiguous images. In the control condition there was no mating cue. In this experiment, they found that the mating cue only worked for male participants, as they were more creative after viewing pictures of attractive females.

Griskevicius, Cialdini and Kenrick (2006) also discussed the difference between short- and long-term relationships, suggesting that females contribute more to short-term relationships whereas males contribute far less, however in a long-term relationship males and females are both likely to invest just as much as each other in the offspring. To test this, they conducted a second experiment where participants in the mating cue condition now imagined either a short-term mating scenario or a long-term mating scenario. These scenarios can be found in Table 3. They also found no difference in increased creativity for females, however, they did find increased creativity for males both in the short-term scenario and the long-term scenario.

In a third study, a long-term committed scenario was added, whereby males and females now both increased creativity in these conditions. According to parental investment theory, this would be because both genders are now providing equal care to the offspring, and so are both putting in effort of creativity (or courtship) for the other gender (Griskevicius, Cialdini & Kenrick, 2006).

Conclusion
Parental investment theory provides an evolutionary perspective into the motives behind why people seek romantic and sexual attraction in a potential partner. In particular, it shows that there is a distinct gender difference between our reasons for engaging in a romantic or sexual relationship. For example, research often finds that males rate their female partners as more attractive and seek young females to continue on their genes. Parental investment theory explains that young females are a good indicator for a male that she is fertile. Females on the other hand, very much rely on the social behaviours of males as indicators that they are good-natured and are willing to care for a child, and so find these behaviours attractive in a male.

Parental investment theory also explains why females seem to be choosier in who they date compared to males. For example, blind date studies in newspaper columns show that females often rated their experience as less enjoyable than the males. This gender difference is explained by the parental investment theory due to the amount of investment each gender needs to give for their offspring. For example, the females have the baby in the womb and nurture it through infancy and so do not have the energy to breed with lots of males, therefore choosing the right male is vital. However, males are not as choosy, and rather rely on the physical attractiveness of the females, and compete for fertile females if not many are available.

However, these gender differences seem to be more important in long-term relationships rather than short-term relationships, as some studies find that the gender difference disappears if males and females are just looking for a short-term relationship. For example, females and males find physical attraction equally important in short-term relationships.

Although parental investment theory has been used in research to understand the gender differences in how each sex is attracted to a mate, there are also questions that have not been answered. Such as, why do females and males develop certain attraction "types" that they desire which do not fit the traditional sex preferences seen in the parental investment theory. Additionally, it does not explain why we are attracted to genetically similar others or same-sex attraction.

Even so, parental investment theory is just one perspective, an evolutionary perspective, about why we are motivated to find romantic and sexual attraction in others. Therefore, males and females differ and tend to be romantically and sexually attracted to someone depending on their parental investment need.