title: Lavaan: how to specify interaction terms in SEM
body: <p>I am using lavaan and have only observed variables (no latent variables).
  I would like to include an interaction term in the model, but not sure how to do this.</p>
  
  <p>This is what I have</p>
  
  <pre><code>model4 &lt;-'
  interac =~ var1 * var2
  Ent ~ age
  presu ~ age + interac
  protein ~ age + fat
  fat ~ age
  tempo ~ age +interac+protein
  score ~sex+education+presu+tempo
  '
  fit &lt;- sem(model4, data=mydata)
  summary(fit4, fit.measures=TRUE)
  </code></pre>
  
  <p>(all variables have been scaled before starting, because I had some issues with some variables being 100 times larger than others).</p>
  
  <p>I am wondering whether this is correct? I don't have the main effects of the interaction in the regression? Shouldn't these be included?
  When I add the interaction term directly in the regression (var1*var2), I get 1 as estimates, so that must be wrong...</p>