This time of year many of us resolve to get back into shape. But if you are having trouble fitting into your tight jeans, it may not be your fault. Scientists now believe that willpower alone is not enough lose weight. They say success depends on your genes, hormones and psychology.

A study of 75 people by BBC Science and Oxford and Cambridge Universities has looked into why one-size-fits-all diets are often not successful. The scientists divided over-eaters into three groups – people who feast, people who constantly crave food and emotional eaters. They tailored diets to the needs of each group.

Feasters can't stop eating once they start. This is because they don't have the hormones that tell them when they are full. Scientists designed a diet for this group featuring high protein foods that make them feel full for a long time. This included fish, chicken, basmati rice and grains. Bread and potatoes were not allowed because they do not fill you up for long.

Constant cravers always feel hungry. Scientists say that certain genes disrupt the messages the stomach sends to the brain saying it is full, meaning the cravers always feel like they need more fatty, sugary food. Dieting seven days a week is very hard for these people, so scientists put them on a normal, healthy diet five days a week, and cut their calorie intake to 800 on two days a week.

Emotional eaters have got into the habit of eating whenever they feel stressed. To help them change this behaviour, scientists offered them group support in meetings and online as well as a diet. During the study, people from all three groups lost weight on the tailored diets, with feasters losing the most and constant cravers losing the least.

So what about you? If any of the eating habits sound familiar, perhaps you should consider a new approach to slimming for 2015.