Interpersonal attraction is the attraction between people which leads to friendships and romantic relationships. The study of interpersonal attraction is a major area of research in social psychology. Interpersonal attraction is related to how much we like, love, dislike, or hate someone. It can be viewed as a force acting between two people that tends to draw them together and resist their separation. When measuring interpersonal attraction, one must refer to the qualities of the attracted as well as the qualities of the attractor to achieve predictive accuracy. It is suggested that to determine attraction, personality and situation must be taken into account. Repulsion is also a factor in the process of interpersonal attraction, one's conception of "attraction" to another can vary from extreme attraction to extreme repulsion.
Many factors leading to interpersonal attraction have been studied. The most frequently studied are: physical attractiveness, propinquity, familiarity, similarity, complementarity, reciprocal liking, and reinforcement.
According to Rowland Miller's Intimate Relationships text, the propinquity effect can be defined as: "the more we see and interact with a person, the more likely he or she is to become our friend or intimate partner." This effect is very similar to the mere exposure effect in that the more a person is exposed to a stimulus, the more the person likes it; however, there are a few exceptions to the mere exposure effect. 
As mentioned above, the mere exposure effect, also known as the familiarity principle, states that the more we are exposed to something, the more we come to like it. This applies equally to both objects and people (Miller, 2006). The social allergy effect occurs when a person's annoying habits grow worse over time, instead of growing more fond of his or her idiosyncrasies.
The notion of “birds of a feather flock together” points out that similarity is a crucial determinant of interpersonal attraction. According to Morry’s attraction-similarity model (2007), there is a lay belief that people with actual similarity produce initial attraction. Perceived similarity develops for someone to rate others as similar to themselves in on-going relationship. Such perception is either self-serving (friendship) or relationship-serving (romantic relationship). Newcomb (1963) pointed out that people tend to change perceived similarity to obtain balance in a relationship. Additionally, perceived similarity was found to be greater than actual similarity in predicting interpersonal attraction.
Findings suggest that interpersonal similarity and attraction are multidimensional constructs (Lydon, Jamieson & Zanna, 1988), in which people are attracted to others who are similar to them in demographics, physical appearance, attitudes, interpersonal style, social and cultural background, personality, interests and activities preferences, and communication and social skills. A study conducted by Theodore Newcomb (1961) on college dorm roommates suggested that individuals with shared backgrounds, academic achievements, attitudes, values, and political views became friends.
Social homogamy refers to “passive, indirect effects on spousal similarity” (Watson et al., 2004, p.1034). The result showed that age and education level are crucial in affecting the mate preference. Because people with similar age study and interact more in the same form of the school, propinquity effect (i.e., the tendency of people to meet and spend time with those who share the common characteristics) plays a significant impact in spousal similarity.
Convergence refers to an increasing similarity with time. Although the previous research showed that there is a greater effect on attitude and value than on personality traits, however, it is found that initial assortment (i.e., similarity within couples at the beginning of marriage), rather than convergence, plays a crucial role in explaining spousal similarity.
Active assortment refers to direct effects on choosing someone similar as self in mating preferences. The data showed that there is a greater effect on political and religious attitudes than on personality traits. A follow-up issue on the reason of the finding was raised. The concepts of idiosyncratic (i.e., different individuals has different mate preferences) and consensual (i.e., a consensus of preference on some prospective mates to others) in mate preference. The data showed that mate preference on political and religious tend to be idiosyncratic, for example, A Catholic prefers to choose the one who is a Catholic, rather than a Buddhist. Such idiosyncratic preference produces high level of active assortment which plays a vital role in affecting spousal similarity.
In summary, active assortment is the most powerful in explaining spousal similarity, whereas convergence has little evidence on showing such effect.
Similarity has effects on starting a relationship by initial attraction to know each other. It is showed that high attitude similarity resulted in a significant increase in initial attraction to the target person and high attitude dissimilarity resulted in a decrease of initial attraction (Gutkin, Gridley & Wendt, 1976; Kaplan & Olczak, 1971). Besides, similarity also promotes relationship commitment. Study on heterosexual dating couples found that similarity in intrinsic values of the couple was linked to relationship commitment and stability (Kurdek & Schnopp-Wyatt, 1997).
The model of complementarity explains whether "birds of a feather flock together" or "opposites attract".
Studies show that complementary interaction between two partners increases their attractiveness to each other (Nowicki and Manheim, 1991). Complementary partners preferred closer interpersonal relationship than non-complementary ones (Nowicki & Manheim,1991). Couples who reported the highest level of loving and harmonious relationship were more dissimilar in dominance than couples who scored lower in relationship quality. (Markey & Markey (2007)).
Mathes and Moore (1985) found that people were more attracted to peers approximating to their ideal self than to those who did not. Specifically, low self-esteem individuals appeared more likely to desire a complementary relationship than high self-esteem people. We are attracted to people who complement to us because this allows us to maintain our preferred style of behavior (Markey & Markey (2007), and through interaction with someone who complements our own behavior, we are likely to have a sense of self-validation and security (Carson, 1969).
Principles of similarity and complementarity seem to be contradictory on the surface (Posavac, 1971; Klohnen & Mendelsohn, 1998). In fact, they agree on the dimension of warmth. Both principles state that friendly people would prefer friendly partners. (Dryer & Horowitz, 1997)
The importance of similarity and complementarity may depend on the stage of the relationship. Similarity seems to carry considerable weight in initial attraction, while complementarity assumes importance as the relationship develops over time (Vinacke, Shannon, Palazzo, Balsavage, et-al, 1988). Markey (2007) found that people would be more satisfied with their relationship if their partners differed from them, at least, in terms of dominance, as two dominant persons may experience conflicts while two submissive individuals may have frustration as neither member take the initiative.
Perception and actual behavior might not be congruent with each other. There were cases that dominant people perceived their partners to be similarly dominant, yet in the eyes of independent observers, the actual behavior of their partner was submissive, in other words, complementary to them (Dryer1997). Why do people perceive their romantic partners to be similar to them despite evidence to the contrary? The reason remains unclear, pending further research.
People's feelings toward a potential partner are dependent on their perception of rewards and costs, the kind of relationships they deserve, and their likelihood for having a healthier relationship with someone else. Rewards are the part of a relationship that makes it worthwhile and enjoyable. A cost is something that can cause irritation like a friend overstaying his welcome. Comparison level is also taken into account during a relationship. This suggests that people expect rewards or costs depending on the time invested in the relationship. If the level of expected rewards are minimal and the level of costs is high, the relationship suffers and both parties may become dissatisfied and unhappy. Lastly, the comparison of alternatives means that satisfaction is conditional on the chance that a person could replace the relationship with a more desirable one.
Warren Kubitschek and Maureen Hallinan, University of Notre Dame, social psychologists who suggested that attraction is the result of the propinquity and similarity effects and the status of each party involved. Their study was about the tracking program that organizes students according to their level of ability to learn. This is mostly implemented in middle and almost all of high school. Their goal is to prove that students on the same track have a higher probability of becoming friends compared to those in different tracks. Other organizational based groupings should also follow these factors. The propinquity effect creates an ideal environment where students are in close physical proximity with each other and have the chance to build familiarity that leads to friendship. Similarity in tracking students is important because they found that track students tend to become friends with others who have the same academic achievement and expectations as themselves. They also found that students on the same level of status concerning grades will likely name them than those who are on lower level than their own. They conclude that although the factors mentioned do have great influence on friendship, they are not exclusive for organized program like tracking.
The triangular theory of love by Robert Sternberg is based on intimacy, passion, and commitment. The strongest type of love, consummate love, consists of these three aspects. The idea of this theory is that love can consist of one component alone or any combination of the three parts: intimacy, passion, and commitment.
There are many factors taken into account when a relationship turns into love. One big factor is culture. This is a common issue among two people who come from very different cultural backgrounds. In a study done by Phillip Shavers and his colleagues, they interviewed participants from different parts of the world and found that love has "similar and different meanings cross-culturally. The Chinese participants had several different love concepts such as "sorrow-love","tenderness-pity", and "sorrow-pity". This ties into another study done by Rothbaym and his partner Tsang in 1998 in which they researched popular love songs from American and Chinese artists. The difference was that the Chinese love songs "had significantly more references to suffering and to negative outcomes than the American love songs". This may be due to beliefs that interpersonal relationships are predestined, and thus have no control over love lives.
The evolutionary theory of human interpersonal attraction states that opposite-sex attraction most often occurs when someone has physical features indicating that he or she is very fertile. Considering that the primary purpose of conjugal/romantic relationships is reproduction, it would follow that people invest in partners who appear very fertile, increasing the chance of their genes being passed down to the next generation. This theory has been criticized because it does not explain relationships between same-sex couples or couples who do not want children, although this may have something to do with the fact that whether one wants children or not one is still subject to the evolutionary forces which produce them.
Another evolutionary explanation suggests that fertility in a mate is of greater importance to men than to women. According to this theory, a woman places significant emphasis on a man's ability to provide resources and protection. The theory suggests that these resources and protection are important in ensuring the successful raising of the woman's offspring. The ability to provide resources and protection might also be sought because the underlying traits are likely to be passed on to male offspring. Critics of this theory point out that most genes are autosomal and non-sex-linked (Gould, et al.)
Evolutionary theory also suggests that people whose physical features suggest they are healthy are seen as more attractive. The theory suggests that a healthy mate is more likely to possess genetic traits related to health that would be passed on to offspring. People's tendency to consider people with facial symmetry more attractive than those with less symmetrical faces is one example. However, a test was conducted that found that perfectly symmetrical faces were less attractive than normal faces. 
It has also been suggested that people are attracted to faces similar to their own. Case studies have revealed that when a photograph of a woman was superimposed to include the features of a man's face, the man whose face was superimposed almost always rated that picture the most attractive. This theory is based upon the notion that we want to replicate our own features in the next generation, as we have survived thus far with such features and have instinctive survival wishes for our children. Another (non-evolutionary) explanation given for the results of that study was that the man whose face was superimposed may have consciously or subconsciously associated the photographically altered female face with the face of his mother or other family member.
A 2009 study by Melissa Burkley and Jessica Parker of Oklahoma State University found that 59% of women stated they were interested in pursuing a relationship when presented with a (unknown to the women) hypothetical "ideal" single man. However, when instead told that the man was already in a romantic relationship, 90% of the women stated they were interested in pursuing a romantic relationship.
Breaking up is the ending of a relationship whether it's a friendship or romantic relationship. There are several reasons that a relationship may come to an end. One reason derives from the equity theory (rewards and costs are equal to both parties). If a person in the relationship feels that the personal costs of being in the relationship outweigh the rewards there is a strong chance that he/she will end the relationship. This also may go for the rewards outweighing costs in some cases. Breaking up can have intense emotional effect on people
A study conducted by Epley and Whitchurch (2008) noted that people often see themselves as more attractive then what they may in fact be. In their experiment on self-recognition, the researchers had 27 Caucasian participants pose for a photograph which was then cropped and morphed into 5 less attractive faces and 5 more attractive faces then the actual photo. After a two to four week period, the participants were invited back to view their photo. They were told that they would be presented with a series of images that had been modified from their original photo. Whilst viewing the 11 photos in a random sequence the participants were asked to point to the one they believed to be their actual face. As predicted, the majority of participants selected one of the more attractive faces as their own, proving a self-serving bias exists when recognising one’s attractiveness.
An article in the Journal of Family Psychology showed that heterosexual relationships in which the man is slightly less attractive than the woman exhibit better interpersonal relationships. The interpretation - or at last one of them - is that one of the things that less attractive men offer to attract more attractive woman with a broader range of choices is greater attentiveness, willingness to listen, etc.