Can the verb “tend” be followed by a bare infinitive (“tend be” v.s. “tend to be”)?

Question

I thought tend (used to imply “regularly or frequently behaving in a particular way or having a certain characteristic – Oxford’s def; 1.1) always has to be used with the to-infinitive form of verbs.

Today, however, I encountered this example:

Most meta-analyses show that, with some exceptions, well-established psychotherapies tend be approximately equivalent in efficacy (Wampold, Mondin, Moody, Stich, Benson, & Ahn, 1997).

And while I couldn’t find anything regarding tend in regards to these two forms (with or without “be”), searching .edu domains precisely for "tend be" returns some 12,000 results. E.g.:

Fungal-like protists and the Chytridiomycota tend be aseptate, except to delimit reproductive or specialized cells (such as sporangia and gametangia).

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The disadvantages of physical sunscreens is that they tend be thicker and look a little white on the skin.

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[The book] reveals that readers [...] tend be more open to diversity; politically tolerant; less authoritarian; less likely to support the use of deadly force or torture; more politically active; and are more likely to have a negative view of the Bush administration.

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It has long been appreciated that genes that perform core functions tend be more conserved in amino acid sequence over long evolutionary time periods.

So, is it ok to write “tends be” instead of “tends to be”?

Answer 1

The OED provides for no such usage. Adapting the examples:

1) Tend can be used with to/towards.

Walter tended to run.

or

Walter tended towards corpulence.

or

... tends towards infinity.

2) Tend can be used with an adverbial:

fire is hot and tends upwards

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No dictionary seems to provide for a usage like:

... tend be ...

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Verdict: typo or perhaps a written form of informal/idiomatic speech. (The "tend be" form sounds slightly familiar to my ear, like I've heard people saying it; particularly in the form "they do tend be ...".)

Answer 2

In addition to badroit's excellent answer:

Tend be is unplottable on an NGRAM.

If you are unfamiliar with its use, an NGRAM is a search for a word form or string within an entire corpus of literature which can compare its relative prevalence (statistical probability of occurrence). When asking a question of this type, plotting an NGRAM can often help answer your question.

Google Books has an excellent NGRAM viewer. It readily allows you to assess the prevalence of words and phrases within all of Google Books's considerable corpus!

As Edwin Ashworth points out, it is not a perfect tool. Just one more in the arsenal.