Stability order of octahydrobiphenylenes I am facing trouble in the following question 


  What is the stability order of the following compounds?
  


In my view cis is less stable than trans. So if prior groups are on the same side of the double bond then it must have less stability. Also conjugated double bond in short non aromatic cyclic system is highly unstable. So 1,2,3,4,5,6,7,8-octahydrobiphenylene (X) should be the least stable. However I could not guess the correct order.
While solving other related questions, I came up with a idea that 


  More is the number of $\alpha$ hydrogen present(hydrogen attached to carbon which is adjacent to a carbon containing double bond ) more stable is the compound.


In the question $Z$ has maximum 10 $\alpha$ hydrogen so it is the most stable.$X$ and $Y$ both have 8 $\alpha$ hydrogen.But $Y$ is more stable since conjugated double bond in short non aromatic cyclic system is highly disfavoured. 

Hence the correct stability order is $Z&gt;Y&gt;X$

Side note- I am still not satisfied with my second argument.Any other explanations or thought will be highly appreciated.