They have discovered two molecules which appear to control the process by which the body creates fat cells.
And they believe genetic mutations to these two molecules may disrupt the mechanism, prompting the body to produce too many or too few fat cells.
The scientists, from Harvard School of Public Health in Cambridge, US, carried out research on mice.
Their findings are reported in the magazine Science.
They found that the animals maintain a pool of cells which can be turned into fat cells when they have unused energy to spare.
They also identified two molecules, known as GATA-2 and GATA-3, which appear to regulate the process by which fat cells are created.
These molecules suppress the process, trapping cells in the "pre-fat" stage.
A failure in their molecular structure may allow the process to spiral out of control.
Conversely, if they are more active, no fat cell formation takes place.
Gateway
Lead researcher Dr Gokhan Hotamisligil compared the process to a gate through which cells had to pass before they could be turned into fat deposits.
He told BBC News Online: "If specific genes form a gate, then slight differences in these genes might explain the differences between "fatness" of individuals.
"If the gate is loose or weak, these individuals will have tendency to become fat easily.
"If it is kept closed strongly, these individuals will be resistant to weight gain."
Dr Hotamisligil said the next stage was to study human genes and the detailed mechanisms operating this gate.
He said: "Understanding these will allow us even better ways to regulate the gate and hence modify fat cell formation and develop novel therapeutics to treat obesity."
Similar findings
The research follows a separate study by a team from the Washington School of Medicine, US, who have identified a gene which helps the body burn off excessive energy.
The Washington researchers believe their discovery could help doctors come up with a treatment for obesity.
The gene in question controls a body chemical called "uncoupling protein one" (UCP1).
When the gene is switched on, the excess energy is released as simple heat rather than turned into the chemical energy that might end up as fat.