Neurotransmitters are not the only chemicals that influence behavior.  Hormones are another important class of chemicals that influence how we think, feel and act - including everything from seeking sex to bonding with a newborn baby.  Take a look at the video below, which introduces some effects of testosterone, ​one of the most well-known hormones.
Video Activity

Watch the video on testosterone below, and answer the following questions:

  • List some of the behaviors that are associated with testosterone

  • How does testosterone explain some of the differences between men and women?

What is testosterone?

Testosterone is one of at least fifty hormones in the human body.  Other well-known hormones include adrenaline, oxytocin, and cortisol.  Unlike neurotransmitters, which are released into the synapse between two neurons in the brain, hormones are produced by specific glands located throughout the body.  These glands make up the endocrine system (see diagram below).  Different hormones are produced by different glands in the body.  Testosterone, for instance, is produced in the male testes (and, to a lesser extent, in the female ovaries).  Hormones are then released into the blood stream, where they have a medium to long term effect on behavior.

Males produce approximately ten times as much testosterone as females do, and this disparity explains many of the behavioral differences between the sexes.  Some of the effects of testosterone on behavior include the following:

  • Male sexual development. All of the changes that occur in boys during puberty - such as voice deepening, increased penis size, muscle development and growth of facial hair - are linked to testosterone.  High levels of testosterone play a role in the male sex drive.

  • Dominance and status seeking. High levels of testosterone are believed to play a role in status seeking behaviors, such as increased competitiveness.  Males with higher levels of testosterone are more likely to be the "alpha males", who thrive on competition, enjoy being the center of attention, and naturally command authority over others

  • Aggression. Since testosterone is linked to status-seeking, it can also play a role in aggression, especially when two males are competing for the same position or resources.  Men with higher levels of testosterone may be more likely to respond violently when they feel their reputation or status is being challenged.  

The following two studies will explore the relationship between testosterone and aggression - the first study was conducted on rats, the second one on humans.
Research Study: Albert 

Aim: Investigate how testosterone influences aggression in "alpha male" rats

Procedure: Rats were placed in cages, and the alpha male rats were identified.  (Alpha males are typically the largest males, who are the most dominant and aggressive).  The alpha males were then randomly assigned to one of four conditions:

A. Castration (i.e. removal of the testicles, which causes a large drop in testosterone)
B. Castration, followed by implanting tubes with testosterone
C. Castration, followed by implanting empty tubes
D. A "sham" operation, in which the rats were cut open & then sewn up, with no castration

Results: The rats whose testosterone levels were diminished (conditions A and C) displayed less aggressive behaviors, such as less biting of other rats.  The rats whose testosterone levels remained the same (conditions B and D) continued to display the same level of aggression. 

Furthermore, when the lowered testosterone rats (conditions A and C) were placed in a cage with another rat, the second rat then took on the role of the "alpha", becoming more aggressive to the rat with the diminished testosterone.

Conclusion: Testosterone plays an important role in aggression and status-seeking in rats.  Higher testosterone increases aggression and dominance of other rats.


  • A carefully controlled laboratory experiment, demonstrating a causal relationship between the independent variable (testosterone levels) and the dependent variable (aggressive and status)

  • This study was performed on rats, so the results may not be completely generalizable to humans.  Status seeking in humans is more complex than in rats - so we can't be sure that testosterone plays the same role
  • This study involved performing permanent surgery on rats, which raises animal rights concerns

Research Study: Dabbs

Aim: Investigate the relationship between testosterone and criminality

Procedure: Testosterone levels were measured by taking saliva samples from 692 male prisoners.  The criminal records of these prisoners were analyzed for violent and non-violent criminal acts

Results: Prisoners with higher testosterone levels were more likely to have committed violent crimes, such as murder, rape, and assault.  Conversely, prisoners with lower testosterone levels were more likely to have committed non-violent crimes, such as credit card fraud

Conclusion: Testosterone appears to be linked to violent, criminal behavior


  • This was a correlational study, as there was no manipulation of any variables.  Therefore, we can't be sure that testosterone causes people to be more violent.  Perhaps it is the other way around - regularly committing acts of violence could increase testosterone levels.  Or maybe there's a hidden third factor - such as growing up in a violent, abusive family - that might be responsible for increasing both testosterone and the likelihood of committing violent crimes
  • This study involved a large sample size, which is a strength of the study.  However, the study was only conducted on males, so we can't be sure if the results also apply to females

Think Critically

A number of studies - on both rats and animals - have found an association between violence and testosterone.  But does that really mean that testosterone causes aggression?  If you gave a normal, healthy man a testosterone patch, would that necessarily make him violent?

Research suggests that the relationship between testosterone and violence is not so simple.  Consider the following:

  • Although violent criminals tend to have higher testosterone than average, so do successful athletes, actors, and business people. 

  • Testosterone facilitate social competition and dominance.  In humans, competing for status isn't necessarily about physical dominance - it could be about who drives the most expensive car or who gets the biggest bonus

  • Rather than testosterone being the cause of violence, it could also be that acting violently causes an increase in testosterone.  We know that testosterone rises in situations of male to male competition - like winning a sports match or a game of chess
IB Psych Matters

Testosterone reaches a peak around the age of 20, and stays high for the next several years.  However, after the age of around 30, testosterone begins to decline in males, by around 1% each year.  Encouraged by the pharmecutical industry, millions of older men now seek out prescibed medication to increase their testosterone levels.

  • List three benefits - and three risks - of increasing testosterone levels
  • You find out that your best male friend is using testosterone cream.  What advice would you give him about the possible effects of increasing testosterone?

  • ​I can explain what a hormone is, and the biological process by which it influences behavior

  • I can describe a number of effects of testosterone on behavior

  • I can describe the Aim, Procedure, Findings, and Conclusion of Albert and Dabbs research on testosterone, and evaluate both studies
  • I can critically evaluate the link between testosterone and aggression
Quiz Yourself!

1.  Where are hormones produced in the body?

(a) In endocrine glands

(b) In endocrine synapses

(c) In reuptake inhibitors

(d) In reuptake neurons

2.  Which statement about the effects of testosterone is false?

(a) Testosterone is associated with increased competitiveness

(b) Testosterone causes growth of facial hair during male puberty

(c) "Alpha males" typically have high levels of testosterone

(d) Men have approximately one hundred times as much testosterone as women

3.  In Albert's study, some rats were assigned to a "sham" condition, in which the rats were simply cut open and then sewn back up.  What effect did this have on testosterone levels?

(a) Testosterone levels increased

(b) Testosterone levels decreased

(c) No change in testosterone levels

(d) The change in testosterone cannot be known

4.  What is the most significant methodological limitation of Dabbs' research?

(a) Small sample size

(b) Unreliability of criminal records

(c) Unreliability of testosterone measurements

(d) Correlational design

5.  In Albert's study, rats with higher testosterone were more physically aggressive.  What is the most convincing reason why these results may not apply to humans?

(a) Rats are biologically more prone to aggression than humans

(b) Social dominance in humans is more complex than in rats
(c) "Alpha males" are only found in animal species, not in humans

(d) It is not ethically possible to castrate humans for research purposes


1 - A, 2 - D, 3 - C, 4 - D, 5 - B