Knockout method

For Male infantile aggression in mice, scientists used a technique called the knockout method, by removing the receptor for the hormone progesterone. Males without the ability to sense the hormone in their system did not attack infants as opposed to unaltered males who regularly attacked infants. Blood tests showed similar levels of the hormone in this group of males and the control that did not endure the knockout of the receptor. The advantage to this method is that you can directly tell if a behavior is hormone controlled by removing the ability to detect the hormone in the specimen as opposed to only measuring hormone levels.

The less invasive method of only measuring hormone levels would not be beneficial for this type of experiment. This is because of the fact both groups had similar concentrations of it, which does not show a direct cause for why one set of males was aggressive and the other was not. By using the knockout method you can find the cause.