Placing two figures side by side [duplicate]

Possible Duplicate:
Two figures side by side
Subfigures placed horizontally

I want to place two figures side by side in my document. I am using the subcaption package and my code is as follows

\documentclass[a4paper, 12pt, notitlepage]{report}
\usepackage{amsfonts} % if you want blackboard bold symbols e.g. for real numbers
\usepackage{graphicx} % if you want to include jpeg or pdf pictures
\usepackage{amsmath}
\usepackage{chngcntr}
\usepackage{wrapfig}
\usepackage{caption}
\usepackage{subcaption}

\begin{document}
\begin{figure}[h]
\centering
\begin{subfigure}{.5\textwidth}
\centering
\includegraphics[width=0.4\textwidth]{tworoundgameeffortvaryingmujnegative}
\caption{J=-1}
\label{fig:tworoundvarymujnegative}
\end{subfigure}
\begin{subfigure}{.5\textwidth}
\centering
\includegraphics[width=0.4\textwidth]{tworoundgameeffortvaryingmujpositive}
\caption{$J=1$}
\label{fig:tworoundvarymujpositive}
\end{subfigure}
\caption{Effort levels in round one and two with varying $\mu$. $\alpha=1$, $v=0.25$ and $w=0.25$}
\label{fig:effortlevelsvarymu}
\end{figure}
\end{document}

For some reason my two pictures get put on top of each other. Does anybody know why this may be the case?

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