I would employ a tabularx environment and use a modified form of the X column type for the first column, where the modification suspends full justification and enables hanging indentation.

\documentclass[12pt]{article} % or some other suitable document class
\usepackage{tabularx,ragged2e,booktabs}
\usepackage{newtxtext,newtxmath}
\newcolumntype{L}{>{\RaggedRight\hangafter=1\hangindent=1em}X}
\newcommand\mc[1]{\multicolumn{2}{c}{#1}}
\begin{document}
\begin{table}
\caption{Participant Characteristics}
\begin{tabularx}{\textwidth}{@{} L r@{\space}l @{\qquad} r@{\space}l c @{}}
\toprule
& \mc{More} & \mc{Less} \\
& \mc{Experienced} & \mc{Experienced} \\
Variables & \mc{Investors} & \mc{Investors} & P-Value$^*$ \\
\midrule
Percentage who had previously invested in the stock market
& \multicolumn{2}{l}{78.50\%} & \multicolumn{2}{l}{36.78\%} & .001 \\
\addlinespace
Years of business work experience
& 6.35 & (6.00) & 1.28 & (2.14) & .001\\
\addlinespace
Age
& 30.46 & (1.70) & 20.11 & (2.67) & .001 \\
\addlinespace
Number of courses taken that discussed investing in mutual funds or the stock market
& 1.58 & (1.70) & .95 & (1.16) & .001 \\
\addlinespace
\dots \\
\bottomrule
\end{tabularx}
\end{table}
\end{document}
\begin{center} \begin{tabular}{lccr} \toprule a & b & c & c' \ \midrule d & e & f & f'\ g & h & i & i'\ l & m & n & n'\ \bottomrule \end{tabular} \end{center}
The table in the image doesn't differ too much from the one produced by the above code. My problem is the length of the entries of the image table, among others.
I made very few tables in my life. I am reading a guide, and I am here to learn from more expert users.
– Vladimir Jul 23 '21 at 14:03