Too often, people who think they're acting assertively are really acting aggressively. The mistake is often accidental. In reality, such people are simply trying to get their needs met, but a huge gap differentiates the two behaviors and a mix-up can bring unintended consequences.
The Oxford English Dictionary tells us that the word assertiveness derives from the verb "to assert," which, according to those Oxford folks, means "to state an opinion, claim a right, or establish authority." They go on to say that if you assert yourself, you "behave in a way that expresses your confidence, importance, or power, and earns you respect from others."
Let's differentiate that from aggressiveness, which means "characterized by aggression: inclined to behave in an actively hostile fashion."
A huge difference can exist between claiming a right and being actively hostile. Which approach brings better results? Your answer may depend on your personality, but those who would like to be seen as aggressive might benefit from learning that Machiavellian management styles are much less effective in the long term.
Essentially, think of assertiveness as being firm, but polite. It's a mindset that says "I want to win, but I'm not going to walk over you to do it - I'm going to respect what you want and work to help you win also."
Aggressiveness, on the other hand, is firm but impolite. The aggressive person says "I'm going to win, and I don't care if you get what you want." Milder forms are more ambivalent: "I don't care whether or not you get your needs met." Full-court press aggressiveness wants the other person to lose no matter what.
By the way, non-assertiveness is polite (considerate of other people's perspectives), but not firm - that is, unwilling to stand up for one's own needs. Non-assertive people need to understand the differences between assertiveness and aggressiveness so that when they do step forward to get their needs met they don't go overboard and step on everyone else.
Usually it's here where the formerly uninitiated says, "Oh, I get it now!" If that's you, congratulations and welcome to the club. To further your understanding, here are a few more differentiators to help you choose assertiveness over aggressiveness:
Aggressive conduct: Glares or stares at others
Assertive conduct: Makes friendly, considerate eye contact
Aggressive conduct: Intimidates others with body language
Assertive conduct: Shows confident body language that matches the message
Aggressive conduct: Has an air of inflexibly—"my way or the highway"
Assertive conduct: States one's needs, but genuinely considers other perspectives
Aggressive conduct: Strives to control others
Assertive conduct: Strives to listen to and work with others
Aggressive conduct: Considers other's perspectives only when demanded to do so
Assertive conduct: Considers other's perspectives without needing to be asked
Aggressive conduct: Values one's self more than others
Assertive conduct: Values self as an equal to others
Aggressive conduct: Will intimidate or even hurt others to avoid being hurt
Assertive conduct: Tries to hurt no one (including self)
Aggressive conduct: Reaches goals usually on the backs of others
Assertive conduct: Strives to reach goals, and help others reach their goals, too
In many ways, assertiveness means standing up for one's self without walking over other people. Those who have been aggressive can gain assertiveness by exercising higher levels of consideration for other people's point of view. This means developing a genuine desire for listening, and not trying to figure someone out by osmosis.
For those who have been non-assertive, assertiveness can be gained by standing up for your own point of view - politeness combined with firmness. This is seeking to get your needs met without backing down like Caspar Milquetoast, yet not crossing the line of walking on other people's needs, which is falling into aggressiveness.
Patience must be developed. Human nature must be considered. The big picture must be seen. Then, with a professionally firm but polite approach, the assertive person earns the respect and cooperation of others much faster and with more commitment than does an aggressive person.
Take it from our friends at the Oxford Dictionary: Assertive people are better able to state an opinion, claim a right, and establish authority. Have a go at it!