Developing a synthesis of relationship principles
The literature on close relationships is dense, with several established models and a lot of experimental data. It's hard to put it all together. There is no single over-arching framework which synthesizes elements identified across multiple research-supported theoretical models.
Scholars Finkel, Simpson and Eastwick (2017) set out to create an integrative perspective on the relationship literature with the goals of refining existing theories, generating new theories, and attempting to move relationship science forward by providing a framework which increases cohesion and reduce conflict among various models. (Warning: there will be jargon, hopefully worth it for folks who like things in plain English, though I've tried to keep it to what is necessary).
Presented here is a synopsis of their framework and findings. The authors review the existing literature, distilling down fourteen core principles of relationship function. The fourteen principles are grouped into four "sets" (see below) based on key organizing questions about what relationships are, how they work, what people bring to relationships as individuals, and how outside factors, such as culture, affect relationships.
The authors incorporate multiple models and cite supporting scientific studies, drawing on attachment theory and interdependence theory as the two most influential over-arching frameworks, in addition to drawing upon risk regulation theory, self-expansion theory, the communal/exchange model, the interpersonal process model of intimacy, and the vulnerability-stress-adaptation model.

In addition to richly cited experimental findings, textbooks, and review articles, the authors refined their meta-framework with review and input from sixteen preeminent relationship researchers in psychology. Much of what the authors review is familiar and immediately relevant. The work is clear and logically organized, with practical implications. The fourteen factors are comprehensive and intuitive, often immediately relevant for thorny relationship situations.
Readers can go to the original publication for more detail and specific references. Future research could examine the factors they have developed to determine if they are fully independent of one another, or break down into fewer factors.
The factors described by Finkel, Simpson and Eastwick are distinct from one another, though interrelated, in how they influence relationships. The fourteen core principles are organized into the sets mentioned above as follows:

1. What is a relationship? Uniqueness, Integration, Trajectory
2. How do relationships operate? Evaluation, Responsiveness, Resolution, Maintenance
3. What tendencies do people bring into their relationships? Predisposition, Instrumentality, Standards
4. How does the context affect relationships? Diagnosticity, Alternatives, Stress, Culture.

 

The fourteen core relationship principles

1. Uniqueness: Relationship outcomes depend not only on the specific qualities of each partner but also on the unique patterns that emerge when the partners’ qualities intersect. Relationships take on a life of their own, arising from but partially independent of the people involved, which influence relationship satisfaction. For example, high levels of mutual commitment lead to better wellness outcomes. In relationships where one person has an anxious attachment style and the other person has an avoidant style, the anxious person is likely to have difficulty talking with the avoidant person about positive things, and thus feel dissatisfied. In relationships where one parter is more neurotic and the other disagreeable (two of the "Big Five" personality dimensions), the neurotic person is more likely to feel depressed during becoming a parent. Many complex factors come together to give each relationship it's unique character.
2. Integration: Opportunities and motivations for interdependence tend to facilitate cognitive, affective, motivational, or behavioral merging between partners. People in close relationships, especially over time, tend to become blended together, losing some sense of individuality as their union evolves. Individual factors such as self-regulation and self-concept may shift, being replaced by mutual regulation and a sense of shared identity springing from the uniqueness of every relationship. Researchers have shown, for example, that when people become closer, they tend to think in more complimentary terms of their partners as people typically do of themselves, which increases the overall self-esteem of the couple through mutually positive synergy. Additionally, couples who collaborate well further one another's goals, individual and shared.
3. Trajectory: The long-term trajectories of relationship dynamics are affected by each partner’s continually updated perceptions of the couple’s relationship-relevant interactions and experiences. Over time relationships change, and hopefully grow rather than petering out or crashing and burning. While various models of relationship change are based on different factors, relationships generally go through developmental stages, analogous to individual development. At each stage, couples navigate different tasks or face new challenges, with opportunities for greater commitment, intimacy and growth along with the hazards of poorly handling difficult times. Passion tends to be stronger earlier in relationships, whereas caregiving and attachment take on greater weight over time.
4. Evaluation: People evaluate their relationships and partners according to a set of positive and negative constructs, which tend to be moderately negatively correlated. We routinely evaluate the world around us, other people, and ourselves. Typically positives and negatives are inversely correlated - when there are more positives, there are less negatives, and visa versa. Relationships can be more difficult if there are high levels of both positive and negative, creating ambivalence. Following Sternberg's triangular theory of love, for example, people may evaluate relationships based on passion, intimacy and commitment. Another influential perspective holds that relationship quality is reflected by evaluation on six dimensions: commitment, trust, love, passion, intimacy, and satisfaction. Recognizing that evaluation has conscious and unconscious components, and considering their impact, can help couples function better and be more satisfied.
5. Responsiveness: Responsive behaviors promote relationship quality for both the self and the partner.  Mutual responsiveness is a key aspect of relationships. Partners in a successful relationship support one another's "core needs and values". The ways that partners are responsive is important as well. For some relationships, responding right away can feel too much like a transaction if on partner is into sharing without expecting something in return, and the other more tit-for-tat. In general, with high mutuality, both partners feel safer and more positive about themselves, and so are willing to be more vulnerable in the relationship, which in turn usually increases closeness. Some attachment styles interact with responsiveness. For example, insecurely attached people may be less responsive when their partners are upset, and when insecurely attached people receive support they may actually feel more insecure as a result. Research has shown that relationships with higher levels of support promote well-being; the individuals involved are happier and healthier than counterparts in less supportive relationships.
6. Resolution: The manner in which partners communicate about and cope with relationship events affects long-term relationship quality and stability. How couples address negative events is crucial to building healthy relationships over time. Negative events have a greater impact than positive events, similar to how people give criticism more weight than praise. How couples deal with conflict is especially important. How couples address conflict can be thought of along two interacting lines: constructive/destructive, and active/passive. Active, constructive conflict management tends to contribute to long-term relationship satisfaction and a lower chance of breaking up. Research from Gottman and Levenson has revealed four behavioral patterns associated with relationships in trouble: "globally criticizing your partner's personality, responding defensively to your partner's criticism, conveying the belief that your partner is beneath you, and refusing to engage with your partner's concerns." Forgiveness is important, and tends to be correlated with better relationship outcomes and growth for both partners - but only when adequate amends are paired with authentic forgiveness.
7. Maintenance: Partners in committed relationships exhibit cognitions and behaviors that promote the relationship’s persistence over time, even if doing so involves self-deceptive biases. Relationships take work to maintain, particularly over longer periods of time. A little bit tricking oneself can go a long way, according to research, though obviously self-deception can go too far. At times, people put aside their own apparent self-interest for the good of the relationship. Of course, tending to the relationship is in their interest, too. People are aware of decisions they make to keep relationships going, but a lot takes place outside of awareness. When people make a commitment to a relationship, they see things differently, and behave correspondingly, including believing their relationships is superior to others, downplaying or foregoing romantic options outside of the relationship, giving things up for the relationship, and letting go of grudges more easily after an offense. Research shows that having an inflated sense of a partner's positive attributes correlates with good relationship outcomes, as does interpreting their behaviors in a more generous light.
8. Predisposition: People bring certain basic qualities of personality and temperament to their relationships, some of which influence their own and their partners’ relationship wellbeing. Even as relationships become integrated over time, the raw materials of the relationship are the strengths and liabilities which the individuals bring to the table. For example, strengths might include resilience, good self-image, or a secure attachment style, and liabilities might including greater neuroticism, difficulty handling rejection, or avoidant ways of coping with life's challenges. The authors note that a communal approach to relationships can help to hold things together through challenging times, and that a neurotic predisposition predicts greater difficulty in relationships.
9. Instrumentality: People bring certain goals and needs to their relationships, and the dynamics between the two partners affect the extent to which they succeed in achieving these goals and meeting these needs. Under good circumstances, relationships further the goals and needs of the individuals involved. Part of the motivation to be in relationships in the first place, therefore, connects with efforts people make to meet those goals and needs. There are obvious goals, such as child rearing and the need for bonding, which are often shared goals. There are others goals which are more individual, and couples in successful relationships typically further one another's pursuits, offering one another help and increasing overall efficiency. Interestingly, research has shown that people in relationships can endure pain better while looking at a photo of their partner, which correlates with brain activity associated with safety. In successful relationships, people find ways to make use of one another, rather than using one another.
10. Standards: People bring certain standards to their relationships and tend to experience greater relationship wellbeing when their relationships exceed these standards. There are several relationship models which look at the role of standards in relationship satisfaction and dysfunction. People commonly recognize that shared values, expectations, desires and ideals are important in relationships. As you might expect, research shows that unrealistically high standards lead to low relationship quality. Likewise, high standards make for better relationships when they increase motivation and support self-improvement efforts - a good relationship brings out the best in both partners.
11. Diagnosticity: Situations vary in the extent to which they afford opportunities to evaluate a partner’s true goals and motives regarding the relationship. Since people tend to assess oneself and others, and the environment they are in, individuals in a relationship are involved in a process of assessing how the relationship is doing and what might be going right - and wrong. Stressful situations really bring out the need to think about the quality of the relationship, and identify problem areas. A "strain test" happens when what is positive for one partner requires a big sacrifice from the other, like a job change which requires one partner move and the other pull up roots. The bigger the sacrifice, the lower the level of connection the sacrificing partner has where they are going, the greater the increase in trust and commitment. Such situations require both partners to make important decisions without full assurance, taking risks in the face of uncertainty, and getting through them successfully supports the couples' perception of having a good relationships they can be proud of, together.
12. Alternatives: The presence of attractive alternatives to a current relationship—including the option of not being in a relationship at all—threatens relationship quality and persistence. Especially when relationships are strained, people consider their alternatives. What could be different in the relationship? Should we stay together? Would I be happier with someone else, someone like _____ who is a friend/co-worker/ex? Would I be better off alone? How we weigh alternatives varies with relationship quality. Highly committed people in satisfying relationships rate alternate mates as less attractive than their peers in troubled relationships, for example. From an evolutionary point of view, when people invest a lot of resources in a relationship, they are more motivated to try to make it work so they don't lose all they've invested. Under such circumstances, we downgrade appealing alternatives, helping keep the relationship safe from infidelity and keep the focus on the current partner. Interestingly, the study authors note that polyamory may decrease the threat of being rejected by permitting alternatives and making them transparent, leading to relationship stability.
13. Stress: High demands external to the relationship predict worse relationship outcomes, especially if the demands exceed the two partners’ (individual or combined) resources for coping. Stressful situations test relationships, and use personal and material resources which otherwise might contribute to a higher quality relationship. Major stressors including unemployment, money issues, going to jail, a serious illness, fertility difficulties, and tragedies like disasters can lead to relationship breakdown and failure. Whether couples use adaptive coping strategies becomes crucial when times are tough, and some coping strategies are more effective than others. Studies show that when personal resources are depleted, people are more likely to be defensive and retaliate when their buttons are pushed. There are many factors which go into how couples may optimally handle high-stake life events.
14. Culture: Relationships are embedded in social networks and a cultural milieu—including norms, practices, and traditions—that shape the nature and trajectory of those relationships. Times change, and societal views shape how people approach relationships and what they are looking for from them. Nowadays, there are so many perspectives on relationships, including the availability of research findings, that people don't always know what cultural influences are at play. Approval or disapproval by friends, family and others can shape the path a relationship takes and how cohesive it becomes. Speculatively, while it seems that approval would hold people together, disapproval might push them apart - or they might band together against disapproving people, possibly at the expense of those relationships. Likewise, cultural and family sexual beliefs and practices strongly influence relationships.

So, those are the 14 core principles in a nutshell. I found it an edifying and organizing schema, and a stimulating read. I am grateful to the authors for synthesizing so much and putting it into a straightforward and commonsense framework.
There's much more to say about each factor, a wealth of data to integrate, and many possibilities for future development. Which factors carry the most weight? What can be modified to have better relationships and avoid common pitfalls? Is there a way to tell at the beginning if a relationship will work out in the long run?
Rather than draw any conclusions at this point, I'm more focused on how having a coherent framework opens up possibilities, and I hope you are, too