Who vs. That

There are many conflicting online sources when it comes to determining whether to use “who” or “that” in a sentence.  However, one rule is absolutely clear:

“Who” should be used only when referring to people. 

“That” can be used for referring to people and objects/subjects. 

For Example:

Correct: The house that became known for its exquisite beauty

Incorrect: The house who became known for its exquisite beauty

There are two very reliable sources that let us know “who” and “that” are interchangeable when it comes to referring to people:

The first is Jean Yates, who says in her book, The Ins and Outs of Prepositions:

An adjective clause can identify a noun.  The clause comes right after the noun. …  TO IDENTIFY A PERSON, AN ADJECTIVE CLAUSE CAN BEGIN WITH WHO(M), THAT, … The man who(m) … The man that …. …  The people who(m) … The people that …

Secondly, the Merriam Webster dictionary, which says of “that”:

a : the person, thing, or idea indicated, mentioned, or understood from the situation b : the time, action, or event specified c : the kind or thing specified as follows d : one or a group of the indicated kind

Using “Who” In The Singular Person

In the singular person, it is preferable to use “who”.  For example:

Peter, who was a fine marksman, shot the thief with expert accuracy. 

Because you wouldn’t say:

Peter, that was a fine marksman, shot the thief with expert accuracy. 

However, you can use “that” on occasion for the singular person.  A writer might do this when referring to someone they don’t know personally or have a name for.  For example:

The guy that came to fix my water pipes last year said the pipes were very worn. 

Using “Who” and “That” In The Plural

In the plural, we can use “that” or “Who”.  For example,

Men that act in this way are quite despicable. 

Men who act in this way are quite despicable. 

As a general rule of thumb use “who” in the singular person, and use “who” and “that” where appropriate in the plural person.  But never use “who” to indicate an object/subject, instead use “that” for that purpose. 

“Who,” rather than “that,” is always used in non-restrictive clauses like “Peter, who was a fine marksman….”  It is not because it is singular, but because it is non-restrictive.  You would say, “The policeman of Chicago, who are noted for their bravery, are not adequately compensated.”  Not “The policemen of Chicago, that are noted for their bravery, are not adequately compensated.”  The number has nothing to do with it.  If the clause is non-restrictive (usually employing commas) use “who” for people and “which” for things.  If the clause is restrictive (and no commas should be used), use either “who” or “that” interchangeably for people, only “that” for other nouns. “ The guy who came to fix my pipes” or “The guy that came to fix my pipes” are equally correct. “ The letter that I wrote to the mayor was published in the paper.”  As opposed to “The letter, which I happened to write while sipping tea, was published in the paper.” 

What about when an entity made up of persons is involved?  I’m reading a research paper for a friend and just read a sentence about U.S. states “who implemented Medicaid programs.”  Is that usage correct? 

Since we are discussing grammar, please note the “faux pas” below in the 2nd sentence.  All commas and periods need to be placed inside quotation marks. 
CORRECT VERSION: In the plural, we can use “that” or “Who.”  For example,
INCORRECT VERSION: In the plural, we can use “that” or “Who”.  For example,
U
sing “Who” and “That” In The Plural