One definition of aggression is behaviour that is intended to cause harm. It can be physical, mental or verbal and in human beings is an unattractive and undesirable trait.

Occasionally it is necessary to seek treatment or undergo therapy in order to moderate over-aggressive behaviour and we have learnt much in recent decades about how various parts of the brain and different hormones can be connected to aggressive behaviour.

Aggression is an action. It is intended to harm someone. It can be a verbal attack--insults, threats, sarcasm, or attributing nasty motives to them--or a physical punishment or restriction. Aggression also seems to be a way of maintaining social order among many species. Animals compete with each other over food, mates, and dwelling spaces, often showing aggression and occurring among virtually all vertebrate species, including humans. However, if aggression is an effective way of maintaining social order, reckless violence appears to be a poor survival mechanism. Nevertheless, this trait has not been wiped out. Since it hasn't disappeared, it is logical that researchers have tried to understand the nature of this behavior. In doing this, there has been an ongoing argument of what its source is.

The nature vs. nurture topic has been a continuing debate for many aspects of human behavior, including aggression. There have been many studies indicating that chemical relationships between serotonin, testosterone, and frontal lobe brain chemistry may play a key role in determining aggressive behavior, while other studies have explored environmental and societal factors that have been said to control patterns in human aggression.

Infoplease defines aggression as behavior that can be directed toward others or internally, and which can be either constructive or destructive. Humans are certainly not alone in expressing aggression against one another. However, aggression in human beings is often motivated by factors beyond physical survival. In many cases, human aggression is not manifested in any physical way.

Physical Violence

Physical aggression often involves acts of violence taken with the intention of causing harm to the recipient, including death, by using weapons or even someone's bare hands. Anger is a frequent source of aggression, but aggressive behavior can also result from intoxication or frustration, according to Wrong Diagnosis and Healthy Children. People suffering from Alzheimer's disease may also manifest aggressive behavior as a result of diminished cognitive capacity, confusion or frustration, according to Healthy Place. Self-mutilation, or physical violence turned against oneself, often occurs in conjunction with serious mental disorders, such as borderline personality disorder, according to the National Institutes of Health and the Associations to Assist Persons With Emotional Lesions as well as antisocial personality disorder, according to OpenMed.

Verbal Hostility

The children's taunt "sticks and stones may break my bones but words will never hurt me" fails to account for emotional abuse carried out through verbal hostility. Verbal aggression includes behavior such as bullying, threats or yelling. The Mayo Clinic includes name-calling and insults under the category of domestic violence. Put-downs, intentional or perceived, can have profound detrimental effects on the recipients. Musician Karen Carpenter reportedly became anorexic after reading a review that called her "chubby." She died in 1983 of complications from anorexia nervosa at only 32 years old, according to Queen City News and OC Weekly.

Nonverbal Intimidation

Nonverbal intimidation often implies the threat of violence, at least in the perception of the person at the receiving end. Stalking often involves one or more forms of nonverbal intimidation, including following the victim, planting malicious software in a victim's computer, sending unwanted gifts and vandalism against the victim's property, according to Sexual Harassment Support. A famous example of nonverbal intimidation occurred during the movie "Fatal Attraction," when Alex kills her victim's daughter's pet rabbit.

Passive Aggression

The Mayo Clinic defines passive aggression as an indirect way of expressing displeasure or anger. Passive aggression is often generated by resentment on the part of someone who is unable or unwilling to express this resentment directly. Deliberately or subconsciously performing a task poorly is one form of passive aggression, agreeing to perform a task but failing to do so is another, according to Psychology Today. Procrastination can also be a form of passive aggression.

There are also certain categories of animal aggression that will not be considered in this introduction, because they are not controlled by the offense system. Some of them may also occur in humans, although they tend to be rare and special occurrences. Four categories should be mentioned.

• Defensive aggression. This is the aggression that may take place when an animal or person is injured or threatened with physical injury. Just as we recognize that a frightened and cornered animal is dangerous, so, too, we recognize that the same phenomenon can occur in people. Defense behavior is analyzed here in the comparative review of muroid rodents.
• Maternal aggression. It is common knowledge that animals defending their young can be especially dangerous to anyone who tries to disturb them, and whether or not this occurs in humans has not yet been scientifically determined. As discussed in the review of muroid rodents, maternal aggression may reflect simultaneous activation of offense and defensive attack.
• Predatory aggression. Carnivorous mammals have a separate system of predatory behavior, based on a brain organization that is separate from offense and defense (see paper on ventromedial tegmental lesions in section on brain mechanisms). Predatory attack may occur on rare occasions in humans; for example, the wild boy of Avignon was said to have killed his prey by a bite to the back of the neck as do other mammalian predators.
• Group display. This behavior which is characteristic of primates, as described in the papers here concerning monkeys, may serve, in some contexts, as an aggressive threat. It apparently represents an independent motivational system, although little is known about its brain organization. It certainly occurs , in humans as well, where its function ranges from the cheering at a sports event to the aggressive cries of a lynch mob.