Self-fulfilling prophesies

I just read a article (https://www.fs.blog/2018/05/pygmalion-effect/) about how our expectations can change the outcomes of what we perceive our students know.  The example give was about horse that read body language and so appeared to count and able to answer questions.  The horse could not do these feats if they could not see the person though, showing that the horse could only do this when it was given social cues.  This is called the Pygmalion Effect. 

In reading this I imagine how this affects what we, as teachers, think our students know.  It is a subconscious bias in thinking our students know more or less than they should or do.  One must be vigilant in how we test and grade our students to be sure we do not do this to them.