Following notes by Phil Ramsden and by Sam Kutler, let me make a few remarks about *constructivism*.
We need to distinguish the WORD *constructivism* from the underlying ideas about how people think. Concerning the WORD -- I don't know exactly how it came into prominence in discussions of mathematics education. In 1984 I published a book (entitled *Learning Mathematics*), and at that time none of the sources, and none of the people, whom I consulted (a very long list) used the word at all. It was never mentioned. In 1990 I co-edited a monograph (Constructivist Views on the Teaching and Learning of Mathematics, published by the NCTM) that (obviously) uses the word in the title (and, in fact, actually uses the word inside the book). So within my personal experience the WORD came into prominence after 1984, and before 1990 -- at least, within the discussions of mathematics education. (I realize that you can find the word used elsewhere in psychology and in education.)
Exactly why the word became fashionable is a mystery to me.
Now, concerning the underlying IDEAS. Of course, Jean Piaget contributed to the underlying ideas in a major way, but he is by no means the earliest writer to deal with these ideas.
In recent years much has been written, mostly on foolish distinctions that will probably not interest most mathematicians, nor many teachers. Even the monograph that I co-edited was, of course, a compromise, and contains some articles that most of us may find more confusing than helpful.
I have never figured out what determines fads and fashions. To me, it is a great mystery. Fo whatever reason, *constructivism* has become a fashionable word, and the underlying ideas (unfortunately often distorted) do get some attention nowadays. I have no doubt -- as Phil Ramsden says - that the basic underlying ideas are correct and valuable.