Centering text in \parbox

I used \parbox to auto-wrap the text, but it should be centered. How can I do?

\begin{table}[H]
\centering
\begin{tabular}{cW{5}W{5}}
\toprule
& {\bf Cost} & {\bf Benefit}\\
\midrule
\multirow{3}{*}{\bf Social} & Possible land competition & Less air pollution \\
& \multirow{2}{*}{\parbox{5cm}{Possible increase of extraction of wood from forest}} & Less medical expenses \\
& & More labour available \\
\bottomrule
\end{tabular}
\end {table}

• try \parbox{5cm}{\centering Possible increase of extraction of wood from forest} Oct 20 '15 at 12:54
• Where is the W column type defined? Oct 20 '15 at 12:56
• There is no need to use parbox here you can simply specify a width instead of * to multirow and it will use a p column. You shouldn't be using \bf in latex (it is not defined by default) Oct 20 '15 at 13:22

I think you are making the markup far more complicated than needed (and please always make your examples complete so people can run them) Something like:

\documentclass{article}
\usepackage{array,booktabs}
\begin{document}

\noindent X \dotfill X

\begin{table}
\centering
\begin{tabular}{c*{2}{>{\centering\arraybackslash}m{4.9cm}}}
\toprule
& \bfseries Cost & \bfseries Benefit\\
\midrule
\bfseries Social
&
Possible land competition

Possible increase of extraction of wood from forest
&
Less air pollution

Less medical expenses

More labour available \\
\bottomrule
\end{tabular}
\end {table}

\end{document}