I've got the head titles constituted by two words and the table is therefore too wide. I tried in word with the same table. If the titles could be split into two lines with each word in a line, the table would be perfectly fixed in. However, I have viewed most of related answers and couldn't work it out in texstudio. I would appreciate a lot if you could help!
\documentclass[a4paper, 12pt]{article}
\usepackage{tabularx}
\usepackage{booktabs}
\usepackage{booktabs}
\usepackage{dcolumn}
\usepackage{multirow}
\usepackage{mathtools}
\usepackage{commath}
\usepackage{amsmath,amssymb}
\DeclareMathOperator{\E}{\mathbb{E}}
\numberwithin{equation}{section}
\usepackage{geometry}
\usepackage{graphicx}
\usepackage{filecontents}
\usepackage[round]{natbib}
\usepackage{pdflscape}
\usepackage{amsfonts}
\usepackage{float}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{filecontents}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\usepackage[short]{optidef}
\usepackage[justification=centering]{caption}
\begin{document}
\begin{table}[htbp]\centering
\def\sym#1{\ifmmode^{#1}\else\(^{#1}\)\fi}
\caption{Demographic Factors for Risky Assets (Pooled Sample) \label{pooledrisky}}
\begin{tabular*}{1\hsize}{@{\hskip\tabcolsep\extracolsep\fill}l*{6}{D{.}{.}{-1}}}
\toprule
&\multicolumn{1}{c}{(1)}&\multicolumn{1}{c}{(2)}&\multicolumn{1}{c}{(3)}&\multicolumn{1}{c}{(4)}&\multicolumn{1}{c}{(5)}&\multicolumn{1}{c}{(6)}\\
&\multicolumn{1}{c}{probit}&\multicolumn{1}{c}{Tobit}&\multicolumn{1}{c}{Tobit\text{ }Censored}&\multicolumn{1}{c}{Tobit \text{ }Truncated}&\multicolumn{1}{c}{Truncated\text{ }Regression}&\multicolumn{1}{c}{Heckman}\\
\midrule
LnInc & 0.0429\sym{***}& 17.28\sym{***}& 1.208 & 2.618 & -1.346 & 0.468 \\
& (0.00425) & (1.863) & (4.635) & (2.932) & (1.132) & (2.500) \\
\addlinespace
Age & -0.000561 & 0.0991 & 0.00693 & 0.0150 & 1.014 & 0.590 \\
& (0.00293) & (1.251) & (0.0914) & (0.190) & (0.796) & (0.672) \\
\addlinespace
Kid & -0.00309 & -2.332 & -0.163 & -0.353 & -2.843 & -1.789 \\
& (0.00926) & (3.992) & (0.685) & (0.722) & (2.907) & (2.040) \\
\addlinespace
Old & 0.0321\sym{***}& 13.36\sym{***}& 0.934 & 2.025 & -0.527 & 0.594 \\
& (0.00992) & (4.260) & (3.597) & (2.348) & (2.861) & (2.352) \\
\addlinespace
HHsize & -0.0210\sym{***}& -8.944\sym{***}& -0.625 & -1.355 & 1.349 & 0.143 \\
& (0.00552) & (2.398) & (2.405) & (1.554) & (1.816) & (1.820) \\
\addlinespace
Male & -0.0103 & -4.335 & -0.303 & -0.657 & 1.060 & 0.257 \\
& (0.00945) & (4.043) & (1.197) & (0.955) & (2.554) & (1.807) \\
\addlinespace
Married & 0.0291\sym{*} & 10.48 & 0.733 & 1.588 & -4.646 & -1.836 \\
& (0.0169) & (7.189) & (2.855) & (2.078) & (4.680) & (4.384) \\
\addlinespace
Party & 0.00880 & 4.225 & 0.295 & 0.640 & 0.101 & 0.269 \\
& (0.0112) & (4.738) & (1.181) & (1.012) & (2.755) & (1.680) \\
\addlinespace
Job & -0.0341\sym{***}& -15.56\sym{***}& -1.088 & -2.358 & -2.167 & -2.262 \\
& (0.0118) & (5.080) & (4.189) & (2.739) & (3.370) & (2.078) \\
\addlinespace
House & 0.0508\sym{***}& 20.74\sym{***}& 1.450 & 3.143 & -1.161 & 0.920 \\
& (0.0158) & (6.798) & (5.584) & (3.652) & (4.567) & (3.741) \\
\addlinespace
Age 25-30 & -0.0393 & -10.86 & -0.759 & -1.645 & 9.263 & 5.502 \\
& (0.0882) & (37.69) & (3.927) & (5.998) & (23.97) & (15.73) \\
\addlinespace
Age 30-35 & 0.0157 & 13.30 & 0.930 & 2.016 & 18.88 & 11.86 \\
& (0.0746) & (31.87) & (4.206) & (5.326) & (20.26) & (14.39) \\
\addlinespace
Age 35-40 & 0.0243 & 16.91 & 1.182 & 2.562 & 17.29 & 11.11 \\
& (0.0598) & (25.56) & (4.875) & (4.812) & (16.27) & (11.79) \\
\addlinespace
Age 40-45 & 0.0442 & 24.83 & 1.736 & 3.763 & 15.12 & 10.42 \\
& (0.0464) & (19.86) & (6.803) & (5.162) & (12.84) & (9.123) \\
\addlinespace
Age 45-50 & 0.0316 & 16.68 & 1.166 & 2.527 & 7.181 & 5.297 \\
& (0.0327) & (14.02) & (4.580) & (3.529) & (9.280) & (5.807) \\
\midrule
Observations & 5174 & 5270 & 5270 & 5270 & 678 & 5270 \\
Log lik. & -1521.7 & -4817.8 & -4817.8 & -4817.8 & -3184.3 & \\
Chi-squared & 975.4 & 981.8 & 981.8 & 981.8 & 54.60 & 59.78 \\
\bottomrule
\multicolumn{7}{l}{\footnotesize Standard errors in parentheses}\\
\multicolumn{7}{l}{\footnotesize \sym{*} \(p<0.1\), \sym{**} \(p<0.05\), \sym{***} \(p<0.01\)}\\
\end{tabular*}
\end{table}
\end{document}

\multicolumnfor the cells with the line-break. And if you have a problem using an already existing answer you should ask a followup question (mostly with link to the already existing one) which shows your problem using the existing answer. In this case we can react on your specific problem. Without it is almost the same question and we would mark it as duplicate. – Schweinebacke Jul 15 '17 at 12:15