The biological approach argues that human attraction has its roots in natural selection – that is, we are attracted to the traits that would have the greatest advantage for our potential offspring.  They also argue that attraction is primarily a physiological response. These physiological responses include neurotransmitters, hormones, and potentially, pheromones.

Much of the research on the biological origins of human attraction has two key limitations.  First, much of the research you will read is correlational in nature - that is, it is not possible to establish a true cause and effect relationship.  Secondly, much of the research is done using animal models, making assumptions about the level to which we can compare human relationships to what we see in the rest of the animal kingdom.

The role of neurotransmitters

A characteristic symptom of romantic love is the obsession with the loved one. Lovers are not able to turn off their thoughts, so most of their waking hours are consumed with thinking about their sweetheart.  This can be observed by the daydreaming of a student in class about his or her new girlfriend, or the incessant texting of two lovebirds when they first meet. When a person is attracted to someone, he or she shows some distinctive features such as an altered mental state with mood swings from depression to joy depending on the response of the loved one.

According to anthropologist Helen Fisher, romantic love should be seen as a motivation system that humans share with other mammals.  Fisher argues that there is a specific attraction system in the brain associated with dopamine-rich areas.  This attraction system has evolved to attract mates and enable individuals to focus their mating energy on one specific partner. This particular system could be characterized as a biochemical cocktail consisting of neurotransmitters such as dopamine, noradrenaline, and serotonin. The overwhelming sense of obsession with the loved one as well as the feelings that your head spins and your heart races, is apparently the result of the 'biochemical cocktail,' according to Fisher et al (2005).

Fisher and her team used an fMRI to test their hypothesis that there are specific neural mechanisms associated with romantic love. The participants consisted of a self-selected sample of 10 women and 7 men who were intensely in love for an average of 7.4 months. The researchers first conducted a semi-structured interview to establish the duration and intensity of the participants’ feelings of romantic love.  Then each participant completed the Passionate Love Scale - a Likert scale questionnaire that measures traits commonly associated with romantic love. This was done in order to compare what participants reported on the questionnaires to their brain activity as seen through the fMRI.

The participants first looked at the photograph of their beloved for 30 seconds while they were scanned. Then they had a filler task to distract them before they looked at a neutral photograph for 30 seconds while being scanned. This was repeated six times. The researchers got what Fisher calls “a beautiful picture of the brain in love”, showing activity in the brain’s reward system. 

The researchers found that the brain’s reward system was particularly active when the lovers looked at pictures of the object of their love  - that is, increased activity in the areas of the brain with high levels of dopamine neurons; they also found that the more passionate they were, the more active the brain’s reward circuitry was.

This is in line with our knowledge of the brain's reward system and the role of dopamine in motivation. The data from the fMRI scans supported a correlation between the attitudes towards the lover and brain activity. According to Fisher, romantic love is not an emotion, but rather a motivation system - a need or a craving - designed to enable lovers to mate. Fisher claims that specific brain systems have evolved to motivate individuals to mate. This could perhaps explain why attraction is normally linked to increased energy, focused attention, obsessive following, sleeplessness, and loss of appetite. Dopamine is behind the intense motivation to win a specific mating partner in the early stages of human romantic love.  In this way, humans are very much like other animals.

 Teacher only box

Think about what role each of the following parts of the brain might play in the experience of attraction.

• The amygdala: encodes the intensity of emotional stimuli whether positive or negative.
• The nucleus accumbens: The nucleus accumbens, being one part of the reward system, plays an important role in processing stimuli that are both rewarding and reinforcing (eg. sex)
• The pre-frontal cortex: can make snap judgments about whether a person is attractive to you.  See this piece of research from Trinity College Dublin.
• The hippocampus: stores and retrieves memories of past experiences.
• The ventral tegmental area (VTA): the dopamine-rich part of the brain that makes us feel good being with the person we love. A good article for further reading can be found here.

Another feature of attraction is the intense craving for the partner. According to research by Marazitti et al. (1999), the obsession by the loved one could perhaps be related to decreased levels of the neurotransmitter serotonin. Marazziti et al. (1999) studied 60 individuals: 20 were men and women who had fallen in love in the previous six months; 20 others suffered from untreated obsessive-compulsive disorder; 20 more were normal, healthy individuals who were not in love - these were used as controls. By analyzing blood samples from the lovers, Marazziti discovered that the serotonin levels of new lovers were equivalent to the low serotonin levels found in people with obsessive-compulsive disorder. Although this is interesting research, it is important to note that researchers analyzed serotonin levels in the blood, not areas of serotonin activity in the brain. Fisher (2005) argues that until scientists have documented the activity of serotonin in specific brain regions, it is not possible to understand the exact role of serotonin in romantic love.

The role of hormones

As a relationship develops over time a couple moves from attraction to a more intimate relationship with feelings of comfort, security, and relatedness called attachment. The British psychiatrist Bowlby (1969) suggested that humans have an innate attachment system that consists of specific behaviours and physiological responses called attachment behaviours. He was mostly concerned with the attachment between a mother and child, but it is believed that the same processes are involved in adult love relationships. Attachment is considered to be fundamental for keeping together two individuals, once the flame of romantic love has vanished.

One hormone that may play a role in long-term commitment is vasopressin, which is released during sex. An experiment on the role of vasopressin was conducted using prairie voles (Winslow et al. 1993). These animals tend to form stable pair bonds and have more sex than is necessary for reproduction, just as humans do. When male prairie voles were given a drug that suppressed the effects of vasopressin, they lost their devotion to their mates and no longer protected them from potential suitors. This was taken as evidence that vasopressin plays an important role in male attachment and mating behaviour.

However, can we generalize from animal research and assume that this is true in human males? Research by Walum et al (2008) argues that it may be possible. They have found that men with a variation of the vasopressor receptor gene (allele 334) which leads to lower levels of vasopressin in human males have lower rates of marital satisfaction.  There are many concerns, however, about arguing that this is “proof” that human monogamous behaviour may be linked to levels of vasopressin.  Firstly, the study is only correlational in nature; unlike the study by Winslow et al, there was no manipulation of the males’ level of vasopressin.  As you can imagine, there would be some pretty serious ethical concerns about such experimentation! Secondly, asking men about their marital satisfaction is open to a lot of confounding variables and demand characteristics.  Finally, arguing that marital satisfaction in a human relationship is equivalent to pair-bonding in prairie voles is problematic. 

A hormone that has received a lot more attention with regard to human mating is oxytocin. Oxytocin is released in both men and women during touching and sex; it is argued that the hormone may intensify feelings of attachment. Thus, couples feel closer and more bonded. Oxytocin is also released during childbirth, and scientists believe that it helps to secure the bond between mother and infant. Research also indicates that oxytocin suppresses the activity of the amygdala, lowering feelings of anxiety and aggression. This may allow a couple to be more intimate and trusting of one another – increasing their bond.

Ditzen et al (2009) performed an experiment with the aim of investigating the possible role of oxytocin in how couples discuss a contentious issue. It was hypothesized that participants who received oxytocin would engage in more positive communication patterns than a placebo group. The researchers used a double-blind placebo-controlled design, with 47 heterosexual couples.  Each couple received either oxytocin or placebo intranasally. They were then videotaped as they engaged in a discussion about a topic that would lead to conflict. The level of the stress hormone cortisol in their saliva was repeatedly measured during the experiment.  The results showed that for both men and women oxytocin improved communication and lowered cortisol, compared to the placebo. These results indicate a possible role of oxytocin in humans and they are in line with animal studies indicating that oxytocin facilitates approach and pair-bonding behaviour. Positive communication in a couple is a necessary factor in a strong relationship – as you will see later in this chapter. However, Ditzen et al. (2009) make clear that it is most likely not the oxytocin level alone that determines the strength of a relationship, but the results are interesting and could perhaps stimulate further research into psychobiological approaches to couple therapy.

 Teacher only box

Do you think that what we see in animals is the same as what we see in humans?  Why or why not?

This returns us to the HL extension and gets students to think about whether we can use animals in the study of human relationships. 

Long-term, monogamous relationships

There are several interesting articles on this.  For example:

• Why pairing up for life is hardly ever a good idea (BBC Earth)
• Are humans meant to be monogamous? (a good comparison of our monogamy to other mammals)
• 10 examples of monogamy in the animal kingdom
• The many forms of monogamy in the animal kingdom

Same-sex relationships

• Are there homosexual animals? (BBC Earth)
• 10 animals that demonstrate homosexual behaviour
• Homosexuality may have evolved for social reasons, not sexual reasons

Grief response after the death of a mate

Perhaps the best article on this is: When animals grieve. The article has several links to research. 

Perhaps the most well-known phenomenon is the "crow funeral."

Evolutionary explanations

Although research on biochemistry helps us to understand what is happening to an individual when falling in love and forming attachments, it does not explain why we find some people more attractive than others. Evolutionary psychologists use the theory of natural selection in order to propose an explanation.

Low (1990) wanted to see if pathogen stress - that is, the exposure to disease and parasites in a community - influences mating behaviour.  He carried out an analysis of 186 cultures and found a strong correlation between the number of parasites the population is exposed to and the degree of polygyny - that is, when males have more than one sexual partner or spouse. As the degree of pathogen stress increases, the number of unmarried men increases. Gangestad and Buss (1993) found that women and men in regions of the world with high levels of pathogen stress rated the importance of physical attractiveness of a prospective mate much more highly than individuals living in regions of the world with lower pathogen stress. For example, symmetrical facial features are considered more attractive in most cultures. In males, the development of prominent cheekbones and a masculine chin is related to androgen levels during puberty. Illness during adolescence can suppress androgen secretion, which affects the development of facial characteristics that are considered attractive. Good teeth may also be an indication of a lower parasite load. Schackelford and Larsen (1997) found that men with fewer symmetrical facial features were less physically active, manifested more symptoms of depression, and reported more minor physical problems (e.g. colds, headaches, gastrointestinal problems).

Another aspect of natural selection would be to make sure that the immune system of any offspring would be as effective as possible. Research on the MHC – or Major Histocompatibility Complex – looks at the role of genes linked to the immune system in human attraction.  MHC has an influence on human smell. Researchers are divided on whether MHC could be an example of a human pheromone.

A female mouse chooses a mate whose major histocompatibility complex (MHC) genes are the least similar to her own.  There is also some indication that human females also prefer men whose MHC genes are the least similar to their own. MHC genes are co-dominant, meaning that both sets of inherited genes have an effect on the child’s immune system; so the more diverse the MHC genes of the parents, the stronger the immune system of the offspring. It would obviously be beneficial, therefore, to have evolved systems of recognizing individuals with different MHC genes, and preferentially selecting them as sexual partners. Wedekind (1995) investigated the role of MHC in human attraction.

Checking for understanding