Article usage: (a/the) mother and child

Question

I am intrigued by these three sentences in dictionaries:

1. Mother and child form a close attachment.
2. Physical contact between a mother and child is very important.
3. Mother and son lost contact when Nicholas was in his early twenties. 

Do the following rewrites sound good? Is there any difference?

4. A mother and child form a close attachment. 
5. Physical contact between mother and child is very important.
6. The mother and son lost contact when Nicholas was in his early twenties. 
7. The mother and the son lost contact when Nicholas was in his early twenties. 

I guess sentence 5 does not work, because physical contact forces us to create the image of two subjects.

A dictionary says that “a” is used once before two nouns that are mentioned together very often, for example, “I'll fetch you a cup and saucer. Does everyone have a knife and fork?” Therefore, I guess it is not good to write “a mother and a child” in sentence 1 and 2.

Answer 1

A mother and child form a close attachment.

Mother and child form a close attachment.

Physical contact between a mother and child is very important.

Physical contact between mother and child is very important.

To me, these two pairs of sentences convey the same meaning.

In each pair, the top one denotes one specific pair of mother-child, while the second one denotes pairs of mother-childs in general.

The mother and son lost contact when Nicholas was in his early twenties.

The mother and the son lost contact when Nicholas was in his early twenties.

To me, the sentence below is more grammatical, as The mother and son feels like a noun phrase; as such, the above sentence has no object (with whom have they lost contact?). Meanwhile, the object of the sentence below is automatically conveyed as both the mother and the son, since lost contact with changes to a reciprocal verb (a verb that affects both parties involved [A → B; B → A]) when and is present.

Unfortunately, I have no other sources other than my ears. However, I do hope this helped!

EDIT/ANSWERED

I came back to this question more than one year later because I felt like at the time I didn't provide a satisfactory answer, and now I think I got it.

This construction is called "coordinated bare singular [nouns]". This paper explains it beautifully (page 3):

(7) Coordinated bare singular noun phrases have a definite meaning.

Thus (6a) is acceptable in just the same way that (8) is acceptable in the same context:

(8) The goblet_i and (the) spoon_h were set on the right of the plate.

As definites, bare singular coordinations require uniqueness. The contrast in (9) is due to the world knowledge that companies may have one president and one vice-president, but typically lots of employees and inspectors.

(9) a. At the company meeting, president and vice-president gave an optimistic speech.

b. ??At the company meeting, employee and inspector talked about their colleagues’ motivation.

In a context in which it can be used anaphorically, the conjunction employee and inspector becomes fine:

(10) Critical speeches were given by a senior employee and a young, dynamic inspector. Inspector and employee agreed that their colleagues’ motivation was too low.

In other cases, the antecedents have not been explicitly mentioned, but they are introduced via bridging from elements in the immediate context, as in these examples from the British National Corpus:

(11) a. The other major role of the sculptor is in the service of religion, where a high degree of interplay between artist and patron is not necessarily so important.

b. The novel (or so I hope) signals a separation between author and narrator with its very first sentence.

Further, coordinated bare singulars cannot appear in existentials with there, in contrast to bare plurals, as illustrated in (12).

(12) a. There were forks on the table.

b. *There were goblet and spoon on the table.

Basically, you have to have context. This construction is really only found in newspaper headlines or articles, where brevity and clarity are important. Otherwise, it isn't used in everyday speech. I hope this answers your question and those of others stumbling on this site looking for an answer!

_{Edit: I recommend that you don't dwell too much on articles. Unlike other Indo-European languages, the importance of articles in English is miniscule.}