Statements like this are all over the Old and New Testament (try Psalms 37, or Matthew 5:5). Mice are a meek lot, but some are meeker than others. How do you tell? You perform the tube test — put two mice at each end of a tube and see which one backs out. If you test groups of 4 mice this way, you find a linear, transitive hierarchy of social dominance 95% of the time. For details see [ Science vol. 334 pp. 608 – 609, 693 and 697 ]. The tube test correlated well with several other tests of social dominance. Transitivity just means that mouse A dominant over mouse B and mouse B dominant over mouse C implies that mouse A is dominant over mouse C. Social dominance is independent of weight, motor skill etc. etc.
So they looked at the way nerve cells in the cerebral cortex of the mice communicate with each other. They studied a type of nerve cell (pyramidal neuron) in the fifth layer in of the medial prefrontal cerebral cortex (which has 6 layers). The pyramidal neurons found in layer V communicate with other parts of the brain. The dominant mice had more effective communication between these cells than the nondominant ones.
Communication between neurons is accomplished by a specialized structure called a synapse where the processes of two nerve cells come into extremely close contact (200 Angstroms) with each other.. Communication between neuron1 (presynaptic) and neuron2 (postsynaptic) is largely one way. Different small molecules (neurotransmitters) are released by presynaptic neurons and bind to proteins in the postsynaptic neuron, which alter their conformation on binding and cause cause the neuron to fire. If this sounds like a drug receptor interaction, well it is. The drug is glutamic acid, a neurotransmitter, and the receptor protein is something called AMPAR (you don’t want to know what the acronym stands for).
So how did the scientists cause the meek to become strong, and the strong to become meek? They injected viruses into the cortex of the animals. The viruses either contained more AMPARs, or a defective AMPAR which bound glutamic acid but didn’t do anything. The viruses were able to get their protein load into the layer five pyramidal neurons. The meek mice getting more AMPARs (so there was more response in the postsynaptic neuron to glutamic acid) became dominant. The dominant mice receiving the defective AMPARs became meek.