Research is the foundation of One Plus One's practical work: monitoring marriage and longterm partnerships, to see how they are changing, what factors cause breakdown, and how people stay together. It undertakes its own original projects and also draws together other research from the UK and abroad, presenting it in a way that policy makers and the public can understand. The nature of its research work informs both national policy and practical initiatives at community level.

One Plus One's research findings are the basis for working projects created by the practice development team to support couples and families. This may involve, for example, training health professionals and others working with families to identify relationship difficulties and to offer effective, acceptable and accessible support to those families. Because its practical projects are grounded in research, they are informed and more likely to be achievable.