Everyone notices when someone gets it wrong, while being perfectly assertive is something that leaders aren't necessarily given credit for. The challenge is made harder by the fact that while observers will frequently agree on someone's level of assertiveness, most individuals are less self-aware: they don't know if others see them as over-assertive or under-assertive.
Being just assertive enough is also complicated by the needs of different situations: the more assertive you are, the more you may get your way, but the less you may get along with others, and vice versa. To a degree, personality dictates assertiveness, but individuals do have behavioural choices. They should think about the costs and benefits of their level of assertiveness, and choose what works best for a particular situation.
Teaming up with someone who complements their style might also increase their chances of successful interactions.
Source: Jerk or wimp: what's your assertiveness style?
Daniel Ames Columbia Ideas at Work, 25 January 2006
Review by Steve Lodge