Impact of learning a new language on productivity

Are there any scientific studies (and conclusions) about the impact of mastering a new programming language over previously learned programming languages? I'm particularly interested if mastering various 开发者_JAVA百科languages can decrease average productivity.

I'm particularly interested if mastering various languages can decrease average productivity.

I strongly doubt this. Since I learned functional programming, I've improved the way I'm writing my Java (non-functional) code as well. Learning new languages poses a great programming exercise that hardly decrease your productivity.