We all know that having good relationships with our family, friends, and partners can make us feel happy and fulfilled. But did you know that they can also improve our physical and mental health? According to decades of research, people who have strong and supportive social ties are more likely to live longer, healthier, and happier lives than those who are lonely or isolated.

One of the most famous and longest-running studies on this topic is the Harvard Study of Adult Development, which has been following the lives of more than 268 Harvard students since 1938. Expanded to include their children and 456 inner-city Boston residents, the study has tracked their health, careers, marriages, and happiness over the course of 80 years, and has found that the quality of their relationships is the most important factor for their well-being. As the director of the study, Robert Waldinger, said in his TED talk, "The surprising finding is that our relationships and how happy we are in our relationships has a powerful influence on our health."

But how exactly do relationships affect our health and happiness? Here are some of the ways that social connections can benefit us:

• Relationships can reduce stress and inflammation. Stress is a major cause of many health problems, such as heart disease, diabetes, and depression. When we are stressed, our body releases a hormone called cortisol, which can damage our cells and organs. However, having close and caring relationships can help us cope with stress and lower our cortisol levels. For example, one study found that married couples who had more positive interactions had lower levels of inflammation than those who had more negative interactions.

• Relationships can enhance our brain function and memory. Our brain needs stimulation and challenge to stay healthy and prevent cognitive decline. Having meaningful conversations, learning new things, and sharing experiences with others can provide us with mental stimulation and boost our brain power.

• Relationships can increase our happiness and life satisfaction. Happiness is not only a pleasant feeling, but also a state of mind that can influence our health and longevity. People who are happy tend to have stronger immune systems, lower blood pressure, and lower risk of chronic diseases. One of the main sources of happiness is having positive and fulfilling relationships with others.

Of course, not all relationships are equally beneficial. Some relationships can be toxic, abusive, or stressful, and can have the opposite effects on our well-being. Therefore, it is important to choose our relationships wisely, and to nurture and maintain them with care and respect.