In men, low testosterone levels in the body can be supplemented by hormone replacement with testosterone. Testosterone replacement therapy can be prescribed as an intramuscular injection usually given on a biweekly basis; as a patch or gel placed on the skin, or as putty that is applied to the gums of the mouth. Each of the treatments has its risks and benefits. The decision as to which form of testosterone to use depends upon the clinical situation. Discussions between the patient and health care professional often helps decide which medication to use.
In the United States there are currently no preparations that are FDA approved for testosterone replacement for women.
Testosterone is significantly correlated with aggression and competitive behaviour and is directly facilitated by the latter. There are two theories on the role of testosterone in aggression and competition.  The first one is the challenge hypothesis which states that testosterone would increase during puberty thus facilitating reproductive and competitive behaviour which would include aggression.  Thus it is the challenge of competition among males of the species that facilitates aggression and violence.  Studies conducted have found direct correlation between testosterone and dominance especially among the most violent criminals in prison who had the highest testosterone levels.  The same research also found fathers (those outside competitive environments) had the lowest testosterone levels compared to other males. 
Are bioidentical hormones natural or not? The answer depends on your outlook. They are manufactured, so if that makes them less than natural in your book they will not fit into the natural supplements for testosterone replacement category for you. However, unlike synthetic hormones, bioidentical hormones have the same molecular structure as hormones created by your body. They can be metabolized and side effects are minimal. Bioidentical hormones are found in topical creams designed to help balance your hormone levels. These creams include products like: