First of all, I'm sure there are no weaknesses in effective listening. Perhaps what you mean is what gets in the way of effective listening?
Listening means really paying attention to what someone is saying and making an effort to look at things from their point of view. Very often we "hear" what someone is saying but make assumptions about what they really mean based on our own beliefs and opinions. Then we get into a bit of verbal shoving match about who is right about this and who is right about that.
Another aspect to effective listening is asking questions. People like to talk about their own experiences and opinions. Asking a few good questions can elicit more information that the person initially provided, which will gratify the speaker and help the listener understand. It shows that you are interested in what they have to say.
Finally, don’t underestimate the importance of body language. The way a person looks and the things they do as they speak gives all kinds of information, often more than the speaker intends to convey.
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The strengths—or "pros"—of effective listening are that one is more likely to understand the sender's perspective, gain insight into a task or situation, or strengthen a relationship than without effective listening. Through effective listening, a person could advance a project past resistance by answering to the concerns others have. An effective listener could gain a promotion at work by paying careful attention to what his or her superiors need. Many a couples therapist has put a broken marriage back together through teaching spouses effective listening. So, in short, effective listening is the key to success in work and relationships!
If there are any weaknesses, or "cons," to effective listening, they would be the same as with any other communication technique. The receiver interprets information based on his or her own understanding. Whatever the intent of the sender, it is possible for the receiver to misinterpret based on his or her own experiences. Language barriers and physical cues—facial expressions, body language—can also affect the receiver's interpretation of a message.
The best way to ensure that effective listening is truly effective is to ask clarifying questions.